Dosing & Uses
Dosage Forms & Strengths
oral solution
- 500mg/5mL
tablet, hippurate
- 1g
tablet, mandelate
- 500mg
- 1g
Urinary Tract Infection Prophylaxis
Indicated for prophylactic or suppressive treatment of frequently recurring urinary tract infections when long-term therapy is considered necessary
Methenamine hippurate: 1 g PO q12hr
Methenamine mandelate: 1 g PO q6hr
Dosing Considerations
Dose selection for an elderly patient should be cautious, usually starting at low end of dosing range, reflecting greater frequency of decreased hepatic, renal or cardiac function, and of concomitant disease or other drug therapy
Dosage Forms & Strengths
oral solution
- 500 mg/5mL
tablet, hippurate
- 1g
tablet, mandelate
- 500mg
- 1g
Urinary Tract Infection Prophylaxis
Indicated for prophylactic or suppressive treatment of frequently recurring urinary tract infections when long-term therapy is considered necessary
Methenamine hippurate
- <6 years: Safety and efficacy not established
- 6-12 years: 0.5-1 g PO q12hr
- >12 years: 1 g PO q12hr
Methenamine mandelate
- <6 years: 18.4 mg/kg PO q6hr
- 6-12 years: 0.5-1 g PO q6hr
- >12 years: 1 g PO q6hr
Interactions
Interaction Checker
No Results
Contraindicated
Serious - Use Alternative
Significant - Monitor Closely
Minor
Contraindicated (0)
Serious - Use Alternative (5)
- acetazolamide
acetazolamide, methenamine. Other (see comment). Contraindicated. Comment: This combination may form an insoluble precipitate in the urine, decreasing the effects of both agents.
- cholera vaccine
methenamine, cholera vaccine. pharmacodynamic antagonism. Avoid or Use Alternate Drug. Avoid coadministration of cholera vaccine with systemic antibiotics since these agents may be active against the vaccine strain. Do not administer cholera vaccine to patients who have received oral or parenteral antibiotics within 14 days prior to vaccination.
- sulfadiazine
methenamine, sulfadiazine. Other (see comment). Contraindicated. Comment: This combination may form an insoluble precipitate in the urine, decreasing the effects of both agents.
- sulfamethoxazole
methenamine, sulfamethoxazole. Other (see comment). Contraindicated. Comment: This combination may form an insoluble precipitate in the urine, decreasing the effects of both agents.
- sulfisoxazole
methenamine, sulfisoxazole. Other (see comment). Contraindicated. Comment: This combination may form an insoluble precipitate in the urine, decreasing the effects of both agents.
Monitor Closely (2)
- pseudoephedrine
methenamine decreases effects of pseudoephedrine by unknown mechanism. Use Caution/Monitor. Urinary excretion of indirect acting alpha/beta agonists (eg, pseudoephedrine) may increase when administered concomitantly with urinary acidifying agents, resulting in lower serum concentrations.
- sodium picosulfate/magnesium oxide/anhydrous citric acid
methenamine decreases effects of sodium picosulfate/magnesium oxide/anhydrous citric acid by altering metabolism. Use Caution/Monitor. Coadministration with antibiotics decreases efficacy by altering colonic bacterial flora needed to convert sodium picosulfate to active drug.
Minor (0)
Adverse Effects
To report suspected adverse reactions, contact Validus Pharmaceuticals LLC at 1-866-982-5438 (1-866-9VALIDUS) or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch
1-10%
Dysuria
Gastric upset
Nausea
Rash
Warnings
Contraindications
Hypersensitivity to methenamine or tartrazine (FD&C Yellow No. 5)
Severe renal or hepatic insufficiency
Concurrent sulfonamides or acetazolamide may form insoluble precipitate in urine
Cautions
Patients with pre-existing hepatic insufficiency may suffer adverse effects from the small amounts of ammonia and formaldehyde that are produced; the classical syndrome of acute hepatic failure may be evoked in these patients
Taking therapy in absence of proven or strongly suspected bacterial infection or a prophylactic indication is unlikely to provide benefit to patient and increases risk of development of drug-resistant bacteria
Large doses of methenamine (8 g daily for 3 to 4 weeks) have caused bladder irritation, painful and frequent micturition, albuminuria, and gross hematuria
Maintain acidic pH of urine, especially when treating urea-splitting organisms, e.g. Proteus, Pseudomonas
Monitor LFTs, especially in patients with history of liver impairment
Safe use not established during pregnancy, especially 1st and 2nd trimester
May precipitate uric acid stones in patients with gout
Pregnancy & Lactation
Pregnancy Category: C
Lactation: excreted in breast milk; no adverse effects documented
Pregnancy Categories
A: Generally acceptable. Controlled studies in pregnant women show no evidence of fetal risk.
B: May be acceptable. Either animal studies show no risk but human studies not available or animal studies showed minor risks and human studies done and showed no risk. C: Use with caution if benefits outweigh risks. Animal studies show risk and human studies not available or neither animal nor human studies done. D: Use in LIFE-THREATENING emergencies when no safer drug available. Positive evidence of human fetal risk. X: Do not use in pregnancy. Risks involved outweigh potential benefits. Safer alternatives exist. NA: Information not available.Pharmacology
Mechanism of Action
Methenamine hydrolyzes to ammonia and formaldehyde (nonspecific antibacterial) in acidic urine
Mandelic acid or hippuric acid acidify the urine
Pharmacokinetics
Half-life: 4.3 hr
Onset: 30 min
Peak plasma time: 3-8 hr
Vd: 0.56 L/kg
Excretion: urine; methenamine 90% excreted w/in 24 hr; mandelic or hippuric acid may accumulate with severe renal impairment
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Formulary
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