Vicks Nyquil (dextromethorphan/pseudoephrine HCl/acetaminophen/doxylamine)

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Contraindicated Drug Combination: This drug combination is contraindicated and generally should not be dispensed or administered to the same patient.

Dextromethorphan/Metaxalone

SEVERITY LEVEL: 1-Contraindicated Drug Combination: This drug combination is contraindicated and generally should not be dispensed or administered to the same patient.

MECHANISM OF ACTION: Dextromethorphan inhibits neural reuptake of serotonin. Metaxalone, a weak inhibitor of MAO, may increase neuronal serotonin concentrations.(1)

CLINICAL EFFECTS: The concurrent use of dextromethorphan with MAOIs may result in hypotension, hyperpyrexia, sedation, somnolence, and death. Symptoms of serotonin syndrome may include tremor, agitation, diaphoresis, hyperreflexia, clonus, tachycardia, hyperthermia, and muscle rigidity.(1)

PREDISPOSING FACTORS: Higher opioid concentrations as may occur due to inhibition of opioid clearance, patient specific genomic factors (e.g. poor metabolizer status for a P450 enzyme), or high opioid dosage may increase the risk for a severe interaction.

PATIENT MANAGEMENT: Dextromethorphan should not be used in patients taking MAOIs such as metaxalone. Use alternative agents for cough. The US manufacturer of Nuedexta (dextromethorphan-quinidine) states Nuedexta is contraindicated within 14 days of MAOI administration.(1) Quinidine increases systemic dextromethorphan concentrations 10 to 20-fold.

DISCUSSION: Metaxalone is a weak inhibitor of MAO.(2,3) There are at least two reports of potential interactions between MAOIs and dextromethorphan. Concomitant use of quinidine, a strong CYP2D6 inhibitor, increases systemic dextromethorphan concentrations 10 to 20-fold. Other strong CYP2D6 inhibitors such as bupropion, fluoxetine and paroxetine could similarly increase dextromethorphan levels and risk for serotonin toxicity in patients also receiving MAOIs.(4,5)

Select Opioids/Sibutramine

SEVERITY LEVEL: 1-Contraindicated Drug Combination: This drug combination is contraindicated and generally should not be dispensed or administered to the same patient.

MECHANISM OF ACTION: Concurrent administration may result in additive effects on serotonin, resulting in serotonin syndrome.

CLINICAL EFFECTS: Concurrent use of sibutramine and certain opioids may result in serotonin syndrome, a serious and potentially fatal reaction. Symptoms of serotonin syndrome may include tremor, agitation, diaphoresis, hyperreflexia, clonus, tachycardia, hyperthermia, and muscle rigidity.(1,3)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: Sibutramine has been removed from the market by the European Medicines Agency and by regulatory agencies in a number of countries including Australia, Canada, Hong Kong, India, Thailand, United Kingdom and the United States. The UK manufacturer of sibutramine formerly stated that sibutramine should not be administered with other drugs that raise serotonin levels in the brain, including certain opioids such as dextromethorphan, meperidine, pentazocine, and fentanyl.(2) The US manufacturer of sibutramine stated that in general, sibutramine should not be administered with other serotonergic agents such as dextromethorphan, meperidine, pentazocine, and fentanyl. However, if concurrent administration is clinically indicated, the patient should be appropriately observed.(1) If concurrent therapy is warranted, patients should be monitored for signs and symptoms of serotonin syndrome. Instruct patients to report muscle twitching, tremors, shivering and stiffness, fever, heavy sweating, heart palpitations, restlessness, confusion, agitation, trouble with coordination, or severe diarrhea. Patients in whom serotonin syndrome is suspected should receive immediate medical attention.

DISCUSSION: Sibutramine has been removed from the market by the European Medicines Agency and by regulatory agencies in a number of countries including Australia, Canada, Hong Kong, India, Thailand, United Kingdom and the United States. Because sibutramine inhibits serotonin reuptake and serotonin syndrome has been reported during concomitant use of selective serotonin reuptake inhibitors and certain opioids such as dextromethorphan, meperidine, pentazocine, and fentanyl, the UK manufacturer of sibutramine states that sibutramine should not be administered with these agents.(2)

Selected Antihistamines/Selected Maois

SEVERITY LEVEL: 1-Contraindicated Drug Combination: This drug combination is contraindicated and generally should not be dispensed or administered to the same patient.

MECHANISM OF ACTION: MAOIs prolong and intensify the effects of antihistamines.(1-6)

CLINICAL EFFECTS: Concurrent use of antihistamines and a MAOI may result in severe hypotension.(1-6)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: Concurrent use of antihistamines and a MAOI is contraindicated.(1-6)

DISCUSSION: MAOIs may prolong and intensify the effects of antihistamines, resulting in severe hypotension.(1-6) A case report describes a patient having cyproheptadine added to their phenelzine therapy in an attempt to relieve the patients anorgasmia. The patient began to suddenly experience visual hallucination after taking the cyproheptadine for two months. Once the medication was terminated, the hallucinations stopped occurring within 48 hours.(7) Methylene blue, when administered intravenously, has been shown to reach sufficient concentrations to be a potent inhibitor of MAO-A.(8,9)

Selected Cyp2d6 Substrates/Mavorixafor

SEVERITY LEVEL: 1-Contraindicated Drug Combination: This drug combination is contraindicated and generally should not be dispensed or administered to the same patient.

MECHANISM OF ACTION: Mavorixafor is a strong inhibitor of CYP2D6 and is expected to inhibit the metabolism of agents through this pathway.(1)

CLINICAL EFFECTS: Concurrent use of mavorixafor may result in elevated levels of and toxicity from agents metabolized by CYP2D6.(1)

PREDISPOSING FACTORS: With tricyclic antidepressants, the risk of seizures may be increased in patients with a history of head trauma or prior seizure; CNS tumor; severe hepatic cirrhosis; excessive use of alcohol or sedatives; addiction to opiates, cocaine, or stimulants; use of over-the-counter stimulants and anorectics; diabetics treated with oral hypoglycemics or insulin; or with concomitant medications known to lower seizure threshold (antipsychotics, theophylline, systemic steroids). With anticholinergic agents, the risk of anticholinergic toxicities including cognitive decline, delirium, falls and fractures is increased in geriatric patients using more than one medicine with anticholinergic properties.(2)

PATIENT MANAGEMENT: The US manufacturer of mavorixafor states concurrent use with CYP2D6 substrate that are highly dependent on CYP2D6 metabolism is contraindicated.(1) The US manufacturer of doxepin states if concurrent use of doxepin and strong CYP2D6 inhibitors such as mavorixafor is warranted, monitor doxepin plasma concentrations and reduce the doxepin dose based on doxepin plasma concentrations.(5)

DISCUSSION: Mavorixafor (400 mg) increased dextromethorphan (CYP2D6 substrate) maximum concentration (Cmax) and area-under-curve (AUC) by 6-fold and 9-fold, respectively.(1) Selected CYP2D6 substrates linked to this monograph include: aripiprazole, atomoxetine, brexpiprazole, desipramine, deutetrabenazine, dextromethorphan, doxepin, encainide, fenfluramine, metoclopramide, methoxyphenamine, metoprolol, mexiletine, nebivolol, paroxetine, perphenazine, risperidone, tetrabenazine, trimipramine, venlafaxine, and yohimbine.

Selected Opioids; Dextromethorphan/Selected Maois

SEVERITY LEVEL: 1-Contraindicated Drug Combination: This drug combination is contraindicated and generally should not be dispensed or administered to the same patient.

MECHANISM OF ACTION: Selected opioids inhibit neural reuptake of serotonin. MAOIs may increase neuronal serotonin concentrations via inhibition of MAO-A.(26)

CLINICAL EFFECTS: The concurrent use of selected opioids with MAOIs has resulted in hypotension, hyperpyrexia, sedation, somnolence, and death. Symptoms of serotonin syndrome may include tremor, agitation, diaphoresis, hyperreflexia, clonus, tachycardia, hyperthermia, and muscle rigidity.(26)

PREDISPOSING FACTORS: Higher opioid concentrations as may occur due to inhibition of opioid clearance, patient specific genomic factors (e.g. poor metabolizer status for a P450 enzyme), or high opioid dosage may increase the risk for a severe interaction.

PATIENT MANAGEMENT: Dextromethorphan, diamorphine, meperidine, and tapentadol should not be used in patients taking MAOIs. Use alternative agents for cough or pain. The US manufacturer of Nuedexta(dextromethorphan-quinidine) states Nuedexta is contraindicated within 14 days of MAOI administration.(28) Quinidine increases systemic dextromethorphan concentrations 10 to 20-fold. Other strong CYP2D6 inhibitors such as bupropion, fluoxetine and paroxetine could similarly increase dextromethorphan levels. The US manufacturer of selegiline states that concurrent use with dextromethorphan or meperidine is contraindicated. The US manufacturers of meperidine and tapentadol and the UK manufacturer of diamorphine state that they should not be used concurrently with or within 14 days of taking an MAOI.

DISCUSSION: The interaction between meperidine and MAOIs has been well documented. There are at least two reports of potential interactions between MAOIs and dextromethorphan. Concomitant use of quinidine, a strong CYP2D6 inhibitor, increases systemic dextromethorphan concentrations 10 to 20-fold. Other strong CYP2D6 inhibitors such as bupropion, fluoxetine and paroxetine could similarly increase dextromethorphan levels and risk for serotonin toxicity in patients also receiving MAOIs. Methylene blue, when administered intravenously, has been shown to reach sufficient concentrations to be a potent inhibitor of MAO-A. One or more of the drug pairs linked to this monograph have been included in a list of interactions that should be considered "high-priority" for inclusion and should not be inactivated in EHR systems. This DDI subset was vetted by an expert panel commissioned by the U.S. Office of the National Coordinator (ONC) for Health Information Technology.

Serotonergic Agents/Dapoxetine

SEVERITY LEVEL: 1-Contraindicated Drug Combination: This drug combination is contraindicated and generally should not be dispensed or administered to the same patient.

MECHANISM OF ACTION: Concurrent administration of dapoxetine with other serotonergic agents may result in additive effects on serotonin, resulting in serotonin syndrome.(1,2)

CLINICAL EFFECTS: Concurrent use of dapoxetine with other serotonergic agents may result in serotonin syndrome, a potentially life-threatening syndrome which may include one or more of the following symptoms: tremor, agitation, diaphoresis, hyperreflexia, clonus, tachycardia, hyperthermia, and muscle rigidity.(3)

PREDISPOSING FACTORS: Use of multiple drugs which increase serotonin levels is associated with an increased risk for this toxidrome.

PATIENT MANAGEMENT: Dapoxetine should not be administered with other serotonergic agents such as the selective serotonin reuptake inhibitors (SSRIs), serotonin-norepinephrine reuptake inhibitors (SNRIs), tricyclic antidepressants (TCAs), L-tryptophan, triptans, tramadol, lithium, and/or St. John's Wort.(1,2) Dapoxetine should not be initiated within 14 days of receiving other serotonergic agents. Other serotonergics should not be initiated within 7 days of receiving dapoxetine.(2)

DISCUSSION: This interaction is based on class labeling for these agents. Although there is no clinical documentation for an interaction between dapoxetine and either the elective serotonin reuptake inhibitors (SSRIs), serotonin-norepinephrine reuptake inhibitors (SNRIs), tricyclic antidepressants (TCAs), L-tryptophan, triptans, tramadol, lithium, and/or St. John's Wort, caution is still warranted. This syndrome has been reported with the selective serotonin reuptake inhibitors and other serotonergic agents, including sumatriptan and dihydroergotamine.

Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

Acetaminophen/Sodium Fusidate

SEVERITY LEVEL: 2-Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

MECHANISM OF ACTION: Sodium fusidate and acetaminophen may inhibit the biotransformation of each other by CYP3A4.(1)

CLINICAL EFFECTS: Concurrent use may result in toxicity from sodium fusidate and acetaminophen.(1)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: The Australian manufacturer of sodium fusidate states that concurrent use of sodium fusidate and CYP3A4 biotransformed drugs such as acetaminophen should be avoided.(1)

DISCUSSION: Although the specifics of sodium fusidate metabolic pathways are not known, an interaction between sodium fusidate and drugs that are biotransformed by CYP3A4 is suspected.(1)

Acetaminophen; Nsaids/Alprostadil

SEVERITY LEVEL: 2-Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

MECHANISM OF ACTION: Alprostadil is a prostaglandin E1 product used to maintain patency of a patent ductus arteriosus (PDA).(1) Acetaminophen and nonsteroidal anti-inflammatory (NSAID) agents inhibit prostaglandins and may be used for PDA closure in addition to pain/fever management.(2-4)

CLINICAL EFFECTS: Simultaneous administration of acetaminophen or NSAIDs may result in decreased clinical effects from alprostadil, including reduction in PDA.(1)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: Avoid concurrent administration of acetaminophen or NSAIDs in patients on alprostadil for maintaining patency of a patent ductus arteriosus (PDA).(1)

DISCUSSION: NSAIDs and acetaminophen are used as management for patent ductus arteriosus (PDA) closure.(2-4) Alprostadil is used to maintain patency of a PDA.(1) In a case report, a 37-week gestational age neonate with cardiac defects required alprostadil therapy for PDA patency. After multiple doses of acetaminophen for pain, an echocardiogram showed reduction of the PDA requiring increased doses of alprostadil. Additional acetaminophen was discontinued. Follow up echocardiogram showed successful reversal of PDA reduction and alprostadil dose was reduced.(5)

Agents That Affect Iodide/Radioactive Iodide

SEVERITY LEVEL: 2-Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

MECHANISM OF ACTION: Many compounds can affect iodide protein binding and alter iodide pharmacokinetics and pharmacodynamics.(1)

CLINICAL EFFECTS: Compounds that affect iodide pharmacokinetics and pharmacodynamics may impact the effectiveness of radioactive iodide.(1)

PREDISPOSING FACTORS: Compounds that affect iodide pharmacokinetics and pharmacodynamics are expected to have the most impact during therapy using radioactive iodide. Diagnostic procedures would be expected to be impacted less.

PATIENT MANAGEMENT: Discuss the use of agents that affect iodide pharmacokinetics and pharmacodynamics with the patient's oncologist.(1) Because indocyanine green contains sodium iodide, the iodine-binding capacity of thyroid tissue may be reduced for at least one week following administration. Do not perform radioactive iodine uptake studies for at least one week following administration of indocyanine green.(2) The manufacturer of iopamidol states administration may interfere with thyroid uptake of radioactive iodine and decrease therapeutic and diagnostic efficacy. Avoid thyroid therapy or testing for up to 6 weeks post administration of iopamidol.(3)

DISCUSSION: Many agents interact with radioactive iodine. The average duration of effect is: anticoagulants - 1 week antihistamines - 1 week anti-thyroid drugs, e.g: carbimazole, methimazole, propylthiouracil - 3-5 days corticosteroids - 1 week iodide-containing medications, e.g: amiodarone - 1-6 months expectorants - 2 weeks Lugol solution - 3 weeks saturated solution of potassium iodine - 3 weeks vitamins - 10-14 days iodide-containing X-ray contrast agents - up to 1 year lithium - 4 weeks phenylbutazone - 1-2 weeks sulfonamides - 1 week thyroid hormones (natural or synthetic), e.g.: thyroxine - 4 weeks tri-iodothyronine - 2 weeks tolbutamide - 1 week topical iodide - 1-9 months (1)

Agents That Affect Iodide/Sodium Iodide I 131

SEVERITY LEVEL: 2-Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

MECHANISM OF ACTION: Many compounds can affect iodide protein binding and alter iodide pharmacokinetics and pharmacodynamics.(1,2)

CLINICAL EFFECTS: Compounds that affect iodide pharmacokinetics and pharmacodynamics may impact the effectiveness of radioactive iodide.(1,2)

PREDISPOSING FACTORS: Compounds that affect iodide pharmacokinetics and pharmacodynamics are expected to have the most impact during therapy using radioactive iodide. Diagnostic procedures would be expected to be impacted less.

PATIENT MANAGEMENT: Discuss the use of agents that affect iodide pharmacokinetics and pharmacodynamics with the patient's oncologist.(1,2) Because indocyanine green contains sodium iodide, the iodine-binding capacity of thyroid tissue may be reduced for at least one week following administration. Do not perform radioactive iodine uptake studies for at least one week following administration of indocyanine green.(3) The manufacturer of iopamidol states administration may interfere with thyroid uptake of radioactive iodine and decrease therapeutic and diagnostic efficacy. Avoid thyroid therapy or testing for up to 6 weeks post administration of iopamidol.(4)

DISCUSSION: Many agents interact with radioactive iodine. The average duration of effect is: anticoagulants - 1 week antihistamines - 1 week anti-thyroid drugs, e.g: carbimazole, methimazole, propylthiouracil - 3-5 days corticosteroids - 1 week iodide-containing medications, e.g: amiodarone - 1-6 months expectorants - 2 weeks Lugol solution - 3 weeks saturated solution of potassium iodine - 3 weeks vitamins - 10-14 days iodide-containing X-ray contrast agents - up to 1 year lithium - 4 weeks phenylbutazone - 1-2 weeks sulfonamides - 1 week thyroid hormones (natural or synthetic), e.g.: thyroxine - 4 weeks tri-iodothyronine - 2 weeks tolbutamide - 1 week topical iodide - 1-9 months (1,2)

Anticholinergics/Clozapine

SEVERITY LEVEL: 2-Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

MECHANISM OF ACTION: Clozapine has potent anticholinergic properties and inhibits serotonin receptors, including 5-HT3.(1-4) Both of these properties may cause inhibition of gastrointestinal (GI) smooth muscle contraction, resulting in decreased peristalsis.(3,4) These effects may be compounded by concurrent use of anticholinergic agents.(1-6)

CLINICAL EFFECTS: Concurrent use of clozapine with other anticholinergic agents may increase the risk of constipation (common) and serious bowel complications (uncommon), including complete bowel obstruction, fecal impaction, paralytic ileus and intestinal ischemia or infarction.(1-6)

PREDISPOSING FACTORS: The risk for serious bowel complications is higher with increasing age, higher frequency of constipation, and in patients on higher doses of clozapine or multiple anticholinergic agents.(1,5)

PATIENT MANAGEMENT: Avoid the use of other anticholinergic agents with clozapine.(1-6) If concurrent use is necessary, evaluate the patient's bowel function regularly. Monitor for symptoms of constipation and GI hypomotility, including having bowel movements less than three times weekly or less than usual, difficulty having a bowel movement or passing gas, nausea, vomiting, and abdominal pain or distention.(2) Consider a prophylactic laxative in those with a history of constipation or bowel obstruction.(2) Review patient medication list for other anticholinergic agents. When possible, decrease the dosage or number of prescribed anticholinergic agents, particularly in the elderly. Counsel the patient about the importance of maintaining adequate hydration. Encourage regular exercise and eating a high-fiber diet.(2)

DISCUSSION: In a prospective cohort study of 26,720 schizophrenic patients in the Danish Central Psychiatric Research Registry, the odds ratio (OR) for ileus was 1.99 with clozapine and 1.48 with anticholinergics. The OR for fatal ileus was 6.73 with clozapine and 5.88 with anticholinergics. Use of anticholinergics with 1st generation antipsychotics (FGA) increased the risk of ileus compare to FGA alone, but this analysis was not done with clozapine.(5) A retrospective cohort study of 24,970 schizophrenic patients from the Taiwanese National Health Insurance Research Database found that the hazard ratio (HR) for clozapine-induced constipation increased from 1.64 when clozapine is used alone, to 2.15 when used concomitantly with anticholinergics. However, there was no significant difference in the HR for ileus when clozapine is used with and without anticholinergics (1.95 and 2.02, respectively).(6) In the French Pharmacovigilance Database, 7 of 38 cases of antipsychotic-associated ischemic colitis or intestinal necrosis involved clozapine, and 5 of these cases involved use of concomitant anticholinergic agents. Three patients died, one of whom was on concomitant anticholinergics.(3) In a case series, 4 of 9 cases of fatal clozapine-associated GI dysfunction involved concurrent anticholinergic agents.(4)

Anticholinergics/Glucagon (DIAGNOSTIC)

SEVERITY LEVEL: 2-Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

MECHANISM OF ACTION: Glucagon and anticholinergic agents may have additive effects on inhibition of gastrointestinal motility.(1)

CLINICAL EFFECTS: Concurrent use of glucagon with anticholinergic agents may increase the risk of gastrointestinal hypomotility, including constipation and bowel complications.(1)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: Concurrent use of glucagon as a diagnotic aid is not recommended with the use of anticholinergic agents.(1) If concurrent use is necessary, evaluate the patient's bowel function. Monitor for symptoms of constipation and gastrointestinal hypomotility.

DISCUSSION: Both glucagon and anticholinergic agents may have additive effects on inhibition of gastrointestinal motility and increase the risk of gastrointestinal adverse effects.(1)

Anticholinergics/Solid Oral Potassium Capsules

SEVERITY LEVEL: 2-Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

MECHANISM OF ACTION: Concentrated potassium may damage the lining of the GI tract. Anticholinergics delay gastric emptying, resulting in the potassium product remaining in the gastrointestinal tract for a longer period of time.(1-16))

CLINICAL EFFECTS: Use of solid oral dosage forms of potassium in patients treated with anticholinergics may result in gastrointestinal erosions, ulcers, stenosis and bleeding.(1-16)

PREDISPOSING FACTORS: Diseases or conditions which may increase risk for GI damage include: preexisting dysphagia, strictures, cardiomegaly, diabetic gastroparesis, elderly status, or insufficient oral intake to allow dilution of potassium.(1-10,21) Other drugs which may add to risk for GI damage include: nonsteroidal anti-inflammatory drugs (NSAIDs), bisphosphonates, or tetracyclines.(21)

PATIENT MANAGEMENT: Regulatory agency and manufacturer recommendations regarding this interaction: - In the US, all solid oral dosage forms (including tablets and extended release capsules) of potassium are contraindicated in patients receiving anticholinergics at sufficient dosages to result in systemic effects.(2-8) Patients receiving such anticholinergic therapy should use a liquid form of potassium chloride.(2) - In Canada, solid oral potassium is contraindicated in any patient with a cause for arrest or delay in tablet/capsule passage through the gastrointestinal tract and the manufacturers recommend caution with concurrent anticholinergic medications.(1,9-10) Evaluate each patient for predisposing factors which may increase risk for GI damage. In patients with multiple risk factors for harm, consider use of liquid potassium supplements, if tolerated. For patients receiving concomitant therapy, assure any potassium dose form is taken after meals with a large glass of water or other fluid. To decrease potassium concentration in the GI tract, limit each dose to 20 meq; if more than 20 meq daily is required, give in divided doses.(2) If concurrent therapy is warranted, monitor patients receiving concurrent therapy for signs of blood loss, including decreased hemoglobin, hematocrit, fecal occult blood, and/or decreased blood pressure and promptly evaluate patients with any symptoms. Patients should be instructed to immediately report any difficulty swallowing, abdominal pain, distention, severe vomiting, or gastrointestinal bleeding. Instruct patients to report any signs and symptoms of bleeding, such as unusual bleeding from the gums or nose; unusual bruising; red or black, tarry stools; red, pink or dark brown urine; acute abdominal or joint pain and/or swelling.

DISCUSSION: In clinical trials, there was a higher incidence of gastric and duodenal lesions in patients receiving a high dose of a wax-matrix controlled-release formulation with a concurrent anticholinergic agent. The lesions were asymptomatic and not accompanied by bleeding, as shown by a lack of positive Hemoccult tests.(1-17) Several studies suggest that the incidence of gastric and duodenal lesions may be less with the microencapsulated formulation of potassium chloride.(14-17)

Anticholinergics/Solid Oral Potassium Tablets

SEVERITY LEVEL: 2-Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

MECHANISM OF ACTION: Concentrated potassium may damage the lining of the GI tract. Anticholinergics delay gastric emptying, resulting in the potassium product remaining in the gastrointestinal tract for a longer period of time.(1-16)

CLINICAL EFFECTS: Use of solid oral dosage forms of potassium in patients treated with anticholinergics may result in gastrointestinal erosions, ulcers, stenosis and bleeding.(1-16)

PREDISPOSING FACTORS: Diseases or conditions which may increase risk for GI damage include: preexisting dysphagia, strictures, cardiomegaly, diabetic gastroparesis, elderly status, or insufficient oral intake to allow dilution of potassium.(1-10,21) Other drugs which may add to risk for GI damage include: nonsteroidal anti-inflammatory drugs (NSAIDs), bisphosphonates, or tetracyclines.(21)

PATIENT MANAGEMENT: Regulatory agency and manufacturer recommendations regarding this interaction: - In the US, all solid oral dosage forms (including tablets and extended release capsules) of potassium are contraindicated in patients receiving anticholinergics at sufficient dosages to result in systemic effects.(2-8) Patients receiving such anticholinergic therapy should use a liquid form of potassium chloride.(2) - In Canada, solid oral potassium is contraindicated in any patient with a cause for arrest or delay in tablet/capsule passage through the gastrointestinal tract and the manufacturers recommend caution with concurrent anticholinergic medications.(1,9-10) Evaluate each patient for predisposing factors which may increase risk for GI damage. In patients with multiple risk factors for harm, consider use of liquid potassium supplements, if tolerated. For patients receiving concomitant therapy, assure any potassium dose form is taken after meals with a large glass of water or other fluid. To decrease potassium concentration in the GI tract, limit each dose to 20 meq; if more than 20 meq daily is required, give in divided doses.(2) If concurrent therapy is warranted, monitor patients receiving concurrent therapy for signs of blood loss, including decreased hemoglobin, hematocrit, fecal occult blood, and/or decreased blood pressure and promptly evaluate patients with any symptoms. Patients should be instructed to immediately report any difficulty swallowing, abdominal pain, distention, severe vomiting, or gastrointestinal bleeding. Instruct patients to report any signs and symptoms of bleeding, such as unusual bleeding from the gums or nose; unusual bruising; red or black, tarry stools; red, pink or dark brown urine; acute abdominal or joint pain and/or swelling.

DISCUSSION: In clinical trials, there was a higher incidence of gastric and duodenal lesions in patients receiving a high dose of a wax-matrix controlled-release formulation with a concurrent anticholinergic agent. Some lesions were asymptomatic and not accompanied by bleeding, as shown by a lack of positive Hemoccult tests.(1-17) Several studies suggest that the incidence of gastric and duodenal lesions may be less with the microencapsulated formulation of potassium chloride.(14-17)

Anticholinergics; Opioids/Eluxadoline

SEVERITY LEVEL: 2-Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

MECHANISM OF ACTION: Eluxadoline is a mixed mu-opioid and kappa-opioid agonist and delta-opioid antagonist and may alter or slow down gastrointestinal transit.(1)

CLINICAL EFFECTS: Constipation related adverse events that sometimes required hospitalization have been reported, including the development of intestinal obstruction, intestinal perforation, and fecal impaction.(1)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: Avoid use with other drugs that may cause constipation. If concurrent use is necessary, evaluate the patient's bowel function regularly. Monitor for symptoms of constipation and GI hypomotility, including having bowel movements less than three times weekly or less than usual, difficulty having a bowel movement or passing gas, nausea, vomiting, and abdominal pain or distention.(1) Instruct patients to stop eluxadoline and immediately contact their healthcare provider if they experience severe constipation. Loperamide may be used occasionally for acute management of severe diarrhea, but must be discontinued if constipation develops.(1)

DISCUSSION: In phase 3 clinical trials, constipation was the most commonly reported adverse reaction (8%). Approximately 50% of constipation events occurred within the first 2 weeks of treatment while the majority occurred within the first 3 months of therapy. Rates of severe constipation were less than 1% in patients receiving eluxadoline doses of 75 mg and 100 mg.(1)

Selected Cyp1a2 Or Cyp2d6 Substrates/Givosiran

SEVERITY LEVEL: 2-Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

MECHANISM OF ACTION: Givosiran interferes with the first and rate-limiting step in hepatic heme biosynthesis, which may lower hepatic heme levels and decrease production and/or activity of cytochrome P450 enzymes.(1,2)

CLINICAL EFFECTS: Concurrent use of givosiran may result in elevated levels of and toxicity from agents metabolized by CYP1A2 or CYP2D6.(1)

PREDISPOSING FACTORS: With tricyclic antidepressants, the risk of seizures may be increased in patients with a history of head trauma or prior seizure; CNS tumor; severe hepatic cirrhosis; excessive use of alcohol or sedatives; addiction to opiates, cocaine, or stimulants; use of over-the-counter stimulants and anorectics; diabetics treated with oral hypoglycemics or insulin; or with concomitant medications known to lower seizure threshold (antipsychotics, theophylline, systemic steroids). With anticholinergic agents, the risk of anticholinergic toxicities including cognitive decline, delirium, falls and fractures is increased in geriatric patients using more than one medicine with anticholinergic properties.(3)

PATIENT MANAGEMENT: Avoid the concurrent use of givosiran with agents that are sensitive substrates of CYP1A2 or CYP2D6, or CYP1A2 or CYP2D6 substrates with a narrow therapeutic index. If concurrent use is unavoidable, decrease the dose of the CYP1A2 or CYP2D6 substrate and monitor patients for toxicity.

DISCUSSION: A study of 9 patients with acute intermittent porphyria found that givosiran decreased the maximum concentration (Cmax) and area-under-curve (AUC) of caffeine (a CYP1A2 substrate) by 1.3- and 3.1-fold, respectively, compared to caffeine alone. Givosiran also decreased the Cmax and AUC of dextromethorphan (a CYP2D6 substrate) by 2- and 2.4-fold, respectively, compared to dextromethorphan alone.(1,2) Selected CYP2D6 substrates linked to this monograph include: atomoxetine, desipramine, deutetrabenazine, dextromethorphan, doxepin, encainide, methoxyphenamine, metoprolol, nebivolol, nefazodone, paroxetine, perphenazine, risperidone, tetrabenazine, trimipramine, and venlafaxine. Selected CYP1A2 substrates linked to this monograph include: agomelatine, aminophylline, rasagiline, tacrine, theophylline, tizanidine, and yohimbine.

Selected Cyp2d6 Substrates/Desvenlafaxine (>100 MG)

SEVERITY LEVEL: 2-Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

MECHANISM OF ACTION: Desvenlafaxine is considered a weak inhibitor of CYP2D6.(1)

CLINICAL EFFECTS: Concurrent use of desvenlafaxine may lead to increased serum levels and adverse effects of drugs sensitive to inhibition of the CYP2D6 pathway.(1) Agents linked to this monograph are: atomoxetine, dapoxetine, deutetrabenazine, dextromethorphan, metoprolol, nebivolol, perphenazine, tolterodine, and yohimbine.

PREDISPOSING FACTORS: With perphenazine and tolterodine, the risk of anticholinergic toxicities including cognitive decline, delirium, falls and fractures is increased in geriatric patients using more than one medicine with anticholinergic properties.(2)

PATIENT MANAGEMENT: Reduce the dose of CYP2D6 substrates by up to one-half when coadministered with desvenlafaxine 400 mg.(1) Studies have shown that desvenlafaxine does not have a clinically relevant effect on CYP2D6 metabolism at the dose of 100 mg daily. CYP2D6 substrates should be dosed at the original level when coadministered with desvenlafaxine 100 mg or lower or when desvenlafaxine is discontinued.(1) The manufacturer of desvenlafaxine does not provide recommendations for use of desvenlafaxine at dosages between 100-400 mg concurrently with CYP2D6 substrates. Monitor for toxicity of the CYP2D6 substrate and consider reducing its dosage.

DISCUSSION: In a study, coadministration of desvenlafaxine 100 mg daily with desipramine (single dose 50 mg) increased desipramine's maximum concentration (Cmax) and area-under-the-curve (AUC)by 25% and 17%.(1) In a study, coadministration of desvenlafaxine 400 mg daily with desipramine (single dose 50 mg) increased desipramine's maximum concentration (Cmax) and area-under-the-curve (AUC)by 50% and 90%.(1) Selected CYP2D6 substrates linked to this monograph are: atomoxetine, dapoxetine, deutetrabenazine, dextromethorphan, metoprolol, nebivolol, perphenazine, tolterodine, and yohimbine.

Selected Cyp2d6 Substrates/Panobinostat

SEVERITY LEVEL: 2-Severe Interaction: Action is required to reduce the risk of severe adverse interaction.

MECHANISM OF ACTION: Panobinostat is a moderate inhibitor of CYP2D6 and is expected to inhibit the metabolism of agents through this pathway.(1)

CLINICAL EFFECTS: Concurrent use of panobinostat may result in elevated levels of and toxicity from agents metabolized by CYP2D6.(1)

PREDISPOSING FACTORS: With tricyclic antidepressants, the risk of seizures may be increased in patients with a history of head trauma or prior seizure; CNS tumor; severe hepatic cirrhosis; excessive use of alcohol or sedatives; addiction to opiates, cocaine, or stimulants; use of over-the-counter stimulants and anorectics; diabetics treated with oral hypoglycemics or insulin; or with concomitant medications known to lower seizure threshold (antipsychotics, theophylline, systemic steroids). With anticholinergic agents, the risk of anticholinergic toxicities including cognitive decline, delirium, falls and fractures is increased in geriatric patients using more than one medicine with anticholinergic properties.(4)

PATIENT MANAGEMENT: Avoid the concurrent use of panobinostat with agents that are sensitive CYP2D6 or CYP2D6 substrates with a narrow therapeutic index. If concurrent use is warranted, monitor patients for toxicity.(1)

DISCUSSION: In a study in 14 subjects with advanced cancer, panobinostat (20 mg daily on Days 3, 5, and 8) increased the maximum concentration (Cmax) and area-under-curve (AUC) of a single dose of dextromethorphan (60 mg) by 20-200% and 20-130%, respectively. Dextromethorphan exposures were extremely variable.(1) Selected CYP2D6 substrates linked to this monograph include: atomoxetine, desipramine, deutetrabenazine, dextromethorphan, doxepin, encainide, methoxyphenamine, metoprolol, nebivolol, paroxetine, perphenazine, risperidone, tetrabenazine, trimipramine, venlafaxine, and yohimbine.

Moderate Interaction: Assess the risk to the patient and take action as needed.

Acetaminophen/Busulfan

SEVERITY LEVEL: 3-Moderate Interaction: Assess the risk to the patient and take action as needed.

MECHANISM OF ACTION: Busulfan is eliminated from the body via glutathione conjugation. Acetaminophen reduces glutathione levels in the blood and tissues and therefore could decrease the elimination rate of busulfan.(1,2)

CLINICAL EFFECTS: Concurrent use of acetaminophen may result in elevated levels of, prolonged exposure to, and toxicity from busulfan, including myelosuppression, granulocytopenia, thrombocytopenia, anemia, seizures, hepatic veno-occlusive disease, cardiac tamponade, bronchopulmonary dysplasia, or cellular dysplasia.(1,2)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: Use acetaminophen concurrent with busulfan with caution.(1) Consider withholding acetaminophen for 72 hours before and during busulfan therapy. If concurrent use cannot be avoided, monitor patients for busulfan toxicity.

DISCUSSION: Although a small population study in adult patients found no effect of acetaminophen on busulfan clearance,(3) caution is still warranted.(1)

Acetaminophen/Isoniazid

SEVERITY LEVEL: 3-Moderate Interaction: Assess the risk to the patient and take action as needed.

MECHANISM OF ACTION: Isoniazid may induce the metabolism of acetaminophen to its toxic N-acetyl-p-benzoquinone imine (NAPQI) metabolite by CYP2E1.(1)

CLINICAL EFFECTS: Concurrent isoniazid and acetaminophen may result in hepatotoxicity.(1) Symptoms can include nausea, vomiting, jaundice, dark urine, abdominal pain, and unexplained fatigue.

PREDISPOSING FACTORS: The interaction may be more severe in fast acetylators.

PATIENT MANAGEMENT: Concurrent use of acetaminophen in patients treated with isoniazid should be approached with caution. Consider an alternative analgesic agent. If concurrent therapy is warranted, advise patients not to exceed the maximum recommended daily dose of acetaminophen and to immediately report any symptoms of hepatotoxicity.

DISCUSSION: Isoniazid has been shown to induce, after initially inhibiting, the metabolism of acetaminophen to N-acetyl-p-benzoquinone imine (NAPQI), which is hepatotoxicity. Normally, NAPQI is rapidly converted to non-toxic metabolites by glutathione; however, high levels of NAPQI can overwhelm this system.(2-4) In a case report, a patient receiving isoniazid developed severe acetaminophen toxicity following a suicide attempt, despite only having ingested a maximum of 11.5 grams of acetaminophen and having a blood acetaminophen level of 15 mmol/L 13 hours later. Toxicity is usually seen with levels greater than 26 mmol/L.(5) In a retrospective review of 20 deaths in patients taking isoniazid alone or with ethambutol during a 13 year period, two deaths involved patients receiving concurrent isoniazid and acetaminophen.(6,7)

Acetaminophen/Selected Anticoagulants

SEVERITY LEVEL: 3-Moderate Interaction: Assess the risk to the patient and take action as needed.

MECHANISM OF ACTION: Acetaminophen may reduce levels of functional Factor VI, thereby increasing the International Normalized Ratio (INR).(1) In one trial factors II and VII levels were also reduced, thereby increasing the INR. (2)

CLINICAL EFFECTS: Concurrent use of routine acetaminophen, especially at dosages greater than 2 grams/day, and coumarin anticoagulants may result in elevated anticoagulant effects.

PREDISPOSING FACTORS: Routine use of acetaminophen at dosages greater than 2 grams/day may increase the risk of the interaction.

PATIENT MANAGEMENT: Patients receiving routine acetaminophen at dosages greater than 2 grams/day with coumarin anticoagulants should be closely monitored for changes in anticoagulant effects. The dosage of the anticoagulant may need to be adjusted. Patients receiving coumarin anticoagulants should be counseled on the use of acetaminophen.

DISCUSSION: A large systematic review was performed on 72 warfarin drug-drug interactions studies that reported on bleeding, thromboembolic events, or death. Most studies were retrospective cohorts. A meta-analysis of 4 of those studies found a higher rate of clinically significant bleeding in patients on warfarin and non-NSAID analgesics (OR=2.12; 95% CI 1.65-2.73). Increased bleeding risk was also seen in subgroup analyses with acetaminophen (OR=2.32; 95% CI 1.22-4.44).(3) In a study in 11 patients maintained on warfarin, use of acetaminophen (4 grams daily for 14 days) increased INR values by an average of 1.04.(4) In a study in 36 patients maintained on warfarin, the addition of acetaminophen (2 grams/day or 4 grams/day) increased INR values.(5) In a study in 20 patients maintained on warfarin, the addition of acetaminophen (4 grams/day for 14 days) increased average INR values by 1.20 (from 2.6 to 3.45).(6) In a study, 12 patients maintained on various anticoagulants (anisindione, dicoumarol, phenprocoumon, and warfarin) who received 4 weeks of acetaminophen (2.6 grams/day) were compared to 50 subjects maintained on various anticoagulants who did not receive acetaminophen. By the third week of concurrent acetaminophen, prothrombin times increased from 23 seconds to 28.4 seconds. The average warfarin-equivalent dose decreased by 5.8 mg to 4.4 mg. In another phase, 50 subjects maintained on various anticoagulants received acetaminophen (2.6 grams/day for 14 days). The mean prothrombin increase was 3.6 seconds.(7) There have been case reports of increased INRs following concurrent acetaminophen in patients maintained on warfarin(8-11) and acenocoumarol.(12) In contrast to the above reports, other studies have found no effects on acenocoumarol,(14) phenprocoumon,(13-15) or warfarin(16,17) by acetaminophen. In a study in 45 patients maintained on warfarin, the addition of acetaminophen (2 or 3 grams/day for 10 days) increased average INR by 0.7 and 0.67 with 2 grams/day and 3 grams/day, respectively. This increase was apparent by day 3, and a decrease in factor II and VII was observed.(2) A self-controlled case study of 1,622 oral anticoagulant-precipitant drug pairs were reviewed and found 14% of drug pairs were associated with a statistically significant elevated risk of thromboembolism. Concurrent use of warfarin and acetaminophen resulted in a ratio of rate ratios (95% CI) of 1.28 (1.18-1.38).(18) One or more of the drug pairs linked to this monograph have been included in a list of interactions that could be considered for classification as "non-interruptive" in EHR systems. This DDI subset was vetted by an expert panel commissioned by the U.S. Office of the National Coordinator (ONC) for Health Information Technology.

Anticholinergics/Topiramate

SEVERITY LEVEL: 3-Moderate Interaction: Assess the risk to the patient and take action as needed.

MECHANISM OF ACTION: Topiramate can cause decreased sweating and elevated body temperature. Agents with anticholinergic activity can predispose patients to heat-related disorders.(1-2)

CLINICAL EFFECTS: Concurrent use of topiramate with agents with anticholinergic activity may increase the incidence of oligohidrosis and hyperthermia, especially in pediatric or adolescent patients.(1-2) Overheating and dehydration can lead to brain damage and death.

PREDISPOSING FACTORS: Pediatric and adolescent patients and patients with dehydration may be more likely to experience heat-related disorders.(1)

PATIENT MANAGEMENT: The manufacturer of topiramate states that caution should be used when topiramate is prescribed with other medicinal products that predispose to heat-related disorders, such as agents with anticholinergic activity (e.g. clomipramine, hydroxyzine, diphenhydramine, haloperidol, imipramine, and oxybutynin) concurrently with zonisamide.(1) Monitor for signs and symptoms of heat stroke: skin feels very hot with little or no sweating, confusion, muscle cramps, rapid heartbeat, or rapid breathing. Monitor for signs and symptoms of dehydration: dry mouth, urinating less than usual, dark-colored urine, dry skin, feeling tired, dizziness, or irritability. If signs or symptoms of dehydration, oligohidrosis, or elevated body temperature occur, discontinuation of zonisamide should be considered.

DISCUSSION: Case reports of decreased sweating and elevated temperature have been reported, especially in pediatric patients. Some cases resulted in heat stroke that required hospital treatment.(1) A 64-year old woman developed non-exertional hyperthemia while taking multiple psychiatric medications with topiramate.(2)

Anticholinergics/Zonisamide

SEVERITY LEVEL: 3-Moderate Interaction: Assess the risk to the patient and take action as needed.

MECHANISM OF ACTION: Zonisamide can cause decreased sweating and elevated body temperature. Agents with anticholinergic activity can predispose patients to heat-related disorders.(1-2)

CLINICAL EFFECTS: Concurrent use of zonisamide with agents with anticholinergic activity may increase the incidence of oligohidrosis and hyperthermia, especially in pediatric or adolescent patients.(1-2) Overheating and dehydration can lead to brain damage and death.

PREDISPOSING FACTORS: Pediatric and adolescent patients and patients with dehydration may be more likely to experience heat-related disorders.(1)

PATIENT MANAGEMENT: The UK and US manufacturers of zonisamide state that caution should be used in adults when zonisamide is prescribed with other medicinal products that predispose to heat-related disorders, such as agents with anticholinergic activity.(1-2) Pediatric and adolescent patients must not take anticholinergic agents (e.g. clomipramine, hydroxyzine, diphenhydramine, haloperidol, imipramine, and oxybutynin) concurrently with zonisamide.(1) Monitor for signs and symptoms of heat stroke: skin feels very hot with little or no sweating, confusion, muscle cramps, rapid heartbeat, or rapid breathing. Monitor for signs and symptoms of dehydration: dry mouth, urinating less than usual, dark-colored urine, dry skin, feeling tired, dizziness, or irritability. If signs or symptoms of dehydration, oligohidrosis, or elevated body temperature occur, discontinuation of zonisamide should be considered.

DISCUSSION: Case reports of decreased sweating and elevated temperature have been reported, especially in pediatric patients. Some cases resulted in heat stroke that required hospital treatment and resulted in death.(1)

Dextromethorphan/Selected Serotonergic Agents

SEVERITY LEVEL: 3-Moderate Interaction: Assess the risk to the patient and take action as needed.

MECHANISM OF ACTION: Dextromethorphan inhibits neuronal reuptake of serotonin. Concurrent administration with one or more serotonergic agents may increase serotonin effects, leading to serotonin toxicity.(1-11)

CLINICAL EFFECTS: The concurrent use of dextromethorphan with serotonergic agents may increase the risk for serotonin syndrome. Serotonin syndrome constitutes a range of toxicities from mild to life threatening.(3) Mild serotonin symptoms may include: shivering, diaphoresis, mydriasis, intermittent tremor, and/or myoclonus.(3) Moderate serotonin symptoms may include: tachycardia, hypertension, hyperthermia, mydriasis, diaphoresis, hyperactive bowel sounds, hyperreflexia, and/OR clonus.(3) Severe serotonin symptoms may include: severe hypertension and tachycardia, shock, agitated delirium, muscular rigidity, and/or hypertonicity.(3)

PREDISPOSING FACTORS: Concurrent use of additional drugs which increase CNS serotonin levels would be expected to further increase risk for serotonin syndrome.(1-11)

PATIENT MANAGEMENT: Monitor patients on multiple serotonergic agents for symptoms of serotonin toxicity. Patients in whom serotonin syndrome is suspected should receive immediate medical attention. If the interacting agents are prescribed by different providers, it would be prudent to assure that both are aware of concomitant therapy and monitoring the patient for serotonin toxicities. Advise patients not to exceed recommended dosages of dextromethorphan. If concurrent therapy is warranted, patients should be monitored for signs and symptoms of serotonin syndrome. Instruct patients to report muscle twitching, tremors, shivering and stiffness, fever, heavy sweating, heart palpitations, restlessness, confusion, agitation, trouble with coordination, or severe diarrhea.

DISCUSSION: Dextromethorphan inhibits neuronal reuptake of serotonin and may potentially precipitate dose-dependant serotonin toxicity in conjunction with other serotonergic agents.(4,5) Serotonin syndrome has been reported in patients following the addition of dextromethorphan containing cough syrups to duloxetine,(6) escitalopram,(7) fluoxetine,(8,9) paroxetine,(10) and sertraline.(11) Selected serotonergic agents linked to this monograph include: centanafadine, citalopram, clomipramine, duloxetine, escitalopram, fluvoxamine, imipramine, levomilnacipran, milnacipran, sertraline, venlafaxine, vilazodone and vortioxetine.

Dextromethorphan/Selected Ssris That Inhibit Cyp2d6

SEVERITY LEVEL: 3-Moderate Interaction: Assess the risk to the patient and take action as needed.

MECHANISM OF ACTION: Fluoxetine and paroxetine, strong inhibitors of CYP2D6, may inhibit the metabolism of dextromethorphan.(1-4)

CLINICAL EFFECTS: Patients may experience increased adverse effects of dextromethorphan due to elevated systemic concentrations. Concomitant use of two or more serotonergic agents increases the risk for serotonin syndrome. Serotonin syndrome constitutes a range of toxicities from mild to life threatening.(5) Mild serotonin symptoms may include: shivering, diaphoresis, mydriasis, intermittent tremor, and/or myoclonus.(5) Moderate serotonin symptoms may include: tachycardia, hypertension, hyperthermia, mydriasis, diaphoresis, hyperactive bowel sounds, hyperreflexia, and/OR clonus.(5) Severe serotonin symptoms may include: severe hypertension and tachycardia, shock, agitated delirium, muscular rigidity, and/or hypertonicity.(5)

PREDISPOSING FACTORS: Concurrent use of additional drugs which increase CNS serotonin levels would be expected to further increase risk for serotonin syndrome.(5)

PATIENT MANAGEMENT: Monitor patients on multiple serotonergic agents for symptoms of serotonin toxicity. Patients in whom serotonin syndrome is suspected should receive immediate medical attention. If the interacting agents are prescribed by different providers, it would be prudent to assure that both are aware of concomitant therapy and monitoring the patient for serotonin toxicities. Advise patients not to exceed recommended dosages of dextromethorphan. If concurrent therapy is warranted, patients should be monitored for signs and symptoms of serotonin syndrome. Instruct patients to report muscle twitching, tremors, shivering and stiffness, fever, heavy sweating, heart palpitations, restlessness, confusion, agitation, trouble with coordination, and/or severe diarrhea.

DISCUSSION: An open label parallel group trial evaluated the interaction between dextromethorphan-quinidine 30 mg-30 mg (higher than marketed strength of 20 mg-10 mg) and paroxetine 20 mg in 27 healthy volunteers with a mean age of 33.6 years. Subjects were randomly divided into 2 groups: - Group 1 received paroxetine 20 mg once daily for 12 days, followed by the addition of dextromethorphan-quinidine twice daily for 8 days. - Group 2 received dextromethorphan-quinidine twice daily for 8 days, followed by paroxetine 20 mg daily for 12 days. Results: overall, adverse effects were reported in 19 of 26 subjects who received combination therapy (73%) and 15 of 27 subjects who received monotherapy (56%). Adverse effects from the combination differed somewhat between groups and were more closely associated with the second drug product administered. Group 1 reported dizziness, headache, somnolence, euphoria, nausea, and vomiting after the addition of dextromethorphan-quinidine to paroxetine. Group 2 adverse events were dizziness, headache, nausea, vomiting, insomnia, anxiety, and hyperhidrosis after the addition of paroxetine to dextromethorphan.(1) Two weeks of fluoxetine therapy increased the area-under-curve (AUC) of dextromethorphan by 27-fold.(4) Serotonin syndrome has been reported in patients following the addition of dextromethorphan containing cough syrups to fluoxetine(6,7) and paroxetine.(8)

Dextromethorphan/Selected Strong Cyp2d6 Inhibitors

SEVERITY LEVEL: 3-Moderate Interaction: Assess the risk to the patient and take action as needed.

MECHANISM OF ACTION: Strong inhibitors of CYP2D6 may inhibit the metabolism of dextromethorphan.(1-4)

CLINICAL EFFECTS: Patients may experience increased adverse effects of dextromethorphan due to elevated systemic concentrations. Elevated levels of dextromethorphan or concomitant use of two or more serotonergic agents increases the risk for serotonin syndrome. Serotonin syndrome constitutes a range of toxicities from mild to life threatening.(5) Mild serotonin symptoms may include: shivering, diaphoresis, mydriasis, intermittent tremor, and/or myoclonus.(5) Moderate serotonin symptoms may include: tachycardia, hypertension, hyperthermia, mydriasis, diaphoresis, hyperactive bowel sounds, hyperreflexia, and/OR clonus.(5) Severe serotonin symptoms may include: severe hypertension and tachycardia, shock, agitated delirium, muscular rigidity, and/or hypertonicity.(5)

PREDISPOSING FACTORS: Concurrent use of additional drugs which increase CNS serotonin levels would be expected to further increase risk for serotonin syndrome.(5)

PATIENT MANAGEMENT: Monitor patients for elevated dextromethorphan levels or on multiple serotonergic agents for symptoms of serotonin toxicity. Patients in whom serotonin syndrome is suspected should receive immediate medical attention. If the interacting agents are prescribed by different providers, it would be prudent to assure that both are aware of concomitant therapy and monitoring the patient for serotonin toxicities. Advise patients not to exceed recommended dosages of dextromethorphan. If concurrent therapy is warranted, patients should be monitored for signs and symptoms of serotonin syndrome. Instruct patients to report muscle twitching, tremors, shivering and stiffness, fever, heavy sweating, heart palpitations, restlessness, confusion, agitation, trouble with coordination, or severe diarrhea.

DISCUSSION: An open label parallel group trial evaluated the interaction between dextromethorphan-quinidine 30 mg-30 mg (higher than marketed strength of 20 mg-10 mg) and paroxetine 20 mg in 27 healthy volunteers with a mean age of 33.6 years. Subjects were randomly divided into 2 groups: - Group 1 received paroxetine 20 mg once daily for 12 days, followed by the addition of dextromethorphan-quinidine twice daily for 8 days. - Group 2 received dextromethorphan-quinidine twice daily for 8 days, followed by paroxetine 20 mg daily for 12 days. Results: overall, adverse effects were reported in 19 of 26 subjects who received combination therapy (73%) and 15 of 27 subjects who received monotherapy (56%). Adverse effects from the combination differed somewhat between groups and were more closely associated with the second drug product administered. Group 1 reported dizziness, headache, somnolence, euphoria, nausea, and vomiting after the addition of dextromethorphan-quinidine to paroxetine. Group 2 adverse events were dizziness, headache, nausea, vomiting, insomnia, anxiety, and hyperhidrosis after the addition of paroxetine to dextromethorphan.(1) Two weeks of fluoxetine therapy increased the area-under-curve (AUC) of dextromethorphan by 27-fold.(4) Serotonin syndrome has been reported in patients following the addition of dextromethorphan containing cough syrups to fluoxetine(6,7) and paroxetine.(8) Selected strong CYP2D6 inhibitors linked to this monograph include: bupropion, dacomitinib, hydroquinidine, quinidine, and terbinafine.(8)

Selected Cyp2d6 Substrates/Desvenlafaxine

SEVERITY LEVEL: 3-Moderate Interaction: Assess the risk to the patient and take action as needed.

MECHANISM OF ACTION: Desvenlafaxine is considered a weak inhibitor of CYP2D6.(1)

CLINICAL EFFECTS: Concurrent use of desvenlafaxine may lead to increased serum levels and adverse effects of drugs sensitive to inhibition of the CYP2D6 pathway.(1) Agents linked to this monograph are: atomoxetine, dapoxetine, deutetrabenazine, dextromethorphan, metoprolol, nebivolol, perphenazine, tolterodine, and yohimbine.

PREDISPOSING FACTORS: With perphenazine and tolterodine, the risk of anticholinergic toxicities including cognitive decline, delirium, falls and fractures is increased in geriatric patients using more than one medicine with anticholinergic properties.(2)

PATIENT MANAGEMENT: Reduce the dose of CYP2D6 substrates by up to one-half when coadministered with desvenlafaxine 400 mg.(1) Studies have shown that desvenlafaxine does not have a clinically relevant effect on CYP2D6 metabolism at the dose of 100 mg daily. CYP2D6 substrates should be dosed at the original level when coadministered with desvenlafaxine 100 mg or lower or when desvenlafaxine is discontinued.(1) The manufacturer of desvenlafaxine does not provide recommendations for use of desvenlafaxine at dosages between 100-400 mg concurrently with CYP2D6 substrates. Monitor for toxicity of the CYP2D6 substrate and consider reducing its dosage.

DISCUSSION: In a study, coadministration of desvenlafaxine 100 mg daily with desipramine (single dose 50 mg) increased desipramine's maximum concentration (Cmax) and area-under-the-curve (AUC)by 25% and 17%.(1) In a study, coadministration of desvenlafaxine 400 mg daily with desipramine (single dose 50 mg) increased desipramine's maximum concentration (Cmax) and area-under-the-curve (AUC)by 50% and 90%.(1) Selected CYP2D6 substrates linked to this monograph are: atomoxetine, dapoxetine, deutetrabenazine, dextromethorphan, metoprolol, nebivolol, perphenazine, tolterodine, and yohimbine.

Serotonergic Agents/Ziprasidone

SEVERITY LEVEL: 3-Moderate Interaction: Assess the risk to the patient and take action as needed.

MECHANISM OF ACTION: Ziprasidone is a 5-HT1A agonist and serotonin and norepinephrine reuptake inhibitor. Concurrent administration with one or more serotonergic agents may increase serotonin effects, resulting in serotonin toxicity.(1,2)

CLINICAL EFFECTS: Concurrent use of ziprasidone and other serotonergic agents may result in serotonin syndrome, a potentially life-threatening condition with symptoms including altered mental status, hypertension, restlessness, myoclonus, hyperthermia, hyperreflexia, diaphoresis, shivering, and tremor.(1)

PREDISPOSING FACTORS: Serotonin syndrome risk is dose-related. Higher systemic concentrations of either drug would be predicted to increase risk for serotonin toxicity.(2) Concomitant therapy with multiple agents which increase brain serotonin concentrations may also increase risk for serotonin syndrome.(2)

PATIENT MANAGEMENT: Caution patients about the risk of serotonin syndrome with the concomitant use of ziprasidone with other serotonergic drugs. Instruct patients to contact their healthcare provider, or report to the emergency room, should they experience signs or symptoms of serotonin syndrome.(1)

DISCUSSION: Several cases of serotonin syndrome have been reported in patients receiving ziprasidone.(4-6)

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• Acetaminophen overdose
• Acute hepatic failure
• Acute hepatitis C

• Angle-closure glaucoma
• Benign prostatic hyperplasia
• Bladder outflow obstruction
• Chronic idiopathic constipation
• Disease of liver
• Stenosing peptic ulcer
• Systemic mastocytosis
• Urinary retention

• Coronary artery disease
• Hypertension
• Hyperthyroidism
• Ocular hypertension
• Protein-calorie malnutrition
• Pyloroduodenal obstruction
• Seizure disorder
• Shock

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Less Frequent

• Abnormal hepatic function tests

Rare or Very Rare

• Acute generalized exanthematous pustulosis
• Acute hepatic failure
• Agranulocytosis
• Allergic dermatitis
• Anaphylaxis
• Angioedema
• Blood dyscrasias
• Drug-induced hepatitis
• Extrasystoles
• Hallucinations
• Hemolytic anemia
• Hypersensitivity drug reaction
• Hypotension
• Laryngeal edema
• Leukopenia
• Maculopapular rash
• Neutropenic disorder
• Seizure disorder
• Stevens-johnson syndrome
• Thrombocytopenic disorder
• Toxic epidermal necrolysis
• Vomiting

More Frequent

• Anticholinergic toxicity
• Dizziness
• Drowsy
• Thick bronchial secretions

Less Frequent

• Acute cognitive impairment
• Muscle weakness
• Sedation

Rare or Very Rare

• Abdominal distension
• Abdominal pain with cramps
• Accidental fall
• Acute abdominal pain
• Acute cognitive impairment
• Agitation
• Anorexia
• Anticholinergic toxicity
• Ataxia
• Blurred vision
• Chest discomfort
• Chills
• Constipation
• Diarrhea
• Diplopia
• Dizziness
• Drowsy
• Dry nose
• Dry throat
• Dyspnea
• Dysuria
• Erythema
• Euphoria
• Excitement
• Fatigue
• Headache disorder
• Hyperhidrosis
• Insomnia
• Irritability
• Maculopapular rash
• Malaise
• Medication overuse headache
• Migraine
• Nausea
• Nausea
• Nervousness
• Nightmares
• Palpitations
• Paresthesia
• Pruritus of skin
• Skin photosensitivity
• Skin rash
• Skin rash
• Symptoms of anxiety
• Tachycardia
• Tinnitus
• Tremor
• Urinary retention
• Urticaria
• Urticaria
• Vertigo
• Visual changes
• Vomiting
• Wheezing
• Xerostomia

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