Hiprex (methenamine hippurate)

Methenamine is commercially available as methenamine hippurate and methenamine mandelate; dosage is expressed in terms of the salt.

No dosing information is available for this tab.

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

Antibiotics/Fecal Microbiota

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: Fecal microbiota is a suspension of live bacteria, which may be compromised by concurrent use of antibiotics.(1)

CLINICAL EFFECTS: Antibiotics may decrease the effectiveness of fecal microbiota.(1)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: Antibiotics should not be used concurrently with fecal microbiota. Antibacterial treatment should be completed for 24 to 72 hours before initiating treatment with fecal microbiota. Do not use antibiotics for up to 8 weeks after fecal microbiota.(1)

DISCUSSION: Antibiotics may compromise the effectiveness of fecal microbiota.

Antibiotics/Fecal Microbiota Spores

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: Fecal microbiota spores is a suspension of live bacterial spores, which may be compromised by concurrent use of antibiotics.(1)

CLINICAL EFFECTS: Antibiotics may decrease the effectiveness of fecal microbiota spores.(1)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: Antibiotics should not be used concurrently with fecal microbiota spores. Antibacterial treatment should be completed for 2 to 4 days before initiating treatment with fecal microbiota spores.(1)

DISCUSSION: Antibiotics may compromise the effectiveness of fecal microbiota spores.

Methenamine/Sulfonamides

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: Methenamine is hydrolyzed to formaldehyde in acidic urine. Sulfonamides may form an insoluble precipitate with formaldehyde in the urine.(1,2)

CLINICAL EFFECTS: The concurrent administration of methenamine and sulfamethizole or sulfathiazole is likely to form a precipitate in the urine.(1-3)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: Methenamine should not be administered to patients receiving sulfonamides.(1-3)

DISCUSSION: Methenamine is hydrolyzed to formaldehyde in acidic urine. An in vitro study showed that addition of methenamine and mandelic acid to saturated solutions of sulfamethizole at pH 5.0 and 6.0 produced a precipitate in one hour.(4)

Methenamine/Urinary Alkalinizers

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: Urinary alkalinizers may elevate urinary ph preventing the conversion of methenamine to formaldehyde and mandelic acid.(1)

CLINICAL EFFECTS: Concurrent administration may result in alkalinization of the urine causing methenamine to be less effective.(1)

PREDISPOSING FACTORS: None determined.

PATIENT MANAGEMENT: The Australian and UK manufacturers of methenamine state that concurrent use with agents that make urine alkaline or urine alkalinizing agents should be avoided.(2,3) Patients receiving concurrent therapy should be monitored for urinary ph and any worsening symptoms of their infection, including dysuria, flank pain, or fever.(1)

DISCUSSION: Administration of urinary alkalinizers may result in alkalinization of the urine resulting in therapeutic failure of methenamine. Formaldehyde is released by acid hydrolysis from methenamine resulting in bactericidal concentrations at urinary ph 5.0 to 5.5. Above urinary ph 6.0 there is insufficient quantities of formaldehyde and methenamine released to achieve a therapeutic response.(1) An in vitro study showed that methenamine efficacy is achieved when the urine pH is less than 5.7 to 5.85.(4)

There are no severe interactions for this drug.

There are no moderate interactions for this drug.

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• Kidney disease with reduction in glomerular filtration rate (GFr)

• Dehydration
• Disease of liver

There are no moderate contraindications for this drug.

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Less Frequent

• Skin rash

Rare or Very Rare

• Hematuria

Less Frequent

• Dyspepsia
• Nausea

Rare or Very Rare

• Dysuria
• Pruritus of skin

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No enhanced messaging available.

Reproduction studies in rats and rabbits using methenamine hippurate have not revealed evidence of harm to the fetus. A slight increase in the stillborn rate and slight impairment of weight gain and survival of live-born offspring was reported in a study in pregnant dogs using oral methenamine in dosages equivalent to the human dosage. There are no adequate and well-controlled studies to date using methenamine hippurate or methenamine mandelate in pregnant women, and the drugs should be used during pregnancy only when clearly needed.

Although safe use of methenamine or its salts during pregnancy has not been definitely established, the drugs have been used in pregnant women without adverse effects to the fetus. One manufacturer of methenamine hippurate states that safety during the last trimester is suggested, but not definitely proven.

The effects of methenamine during labor and delivery are unknown and there are no recognized uses for the drug during labor or delivery.

Because methenamine is distributed into milk and because of the potential for serious adverse effects in nursing infants, a decision should be made whether to discontinue nursing or the drug, taking into account the importance of the drug to the woman.

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