Uses
Urinary Tract Infections (UTIs)
Prophylaxis or suppression of recurrent UTIs (bacteriuria), especially when long-term therapy is considered necessary.
Use only after UTI has been eradicated by other appropriate anti-infectives.
Not effective in systemic bacterial infections and has no effect on bacteria in blood or tissues outside the urinary tract. Do not use alone in the treatment of acute parenchymal infections causing systemic symptoms (e.g., chills, fever).
Monitor efficacy by periodic urine cultures.
Antibacterial effects are maximal when urine pH is ≤5.5. Monitor urinary pH during therapy; use supplementary acidification, if required. Supplementary acidification may be achieved by dietary regulation and/or concomitant administration of acidifying agents (e.g., ammonium chloride, ascorbic acid, methionine). This is particularly important when the causative organisms are urea-splitting strains of Proteus or Pseudomonas which increase urinary pH.
Dosage and Administration
General
- Acidic urine is essential for antibacterial activity; maximum efficacy occurs when urine pH is ≤5.5. Restrict alkalinizing food and medication; use supplemental acidification if needed.
Administration
Oral Administration
Administer orally.
Dosage
Available as methenamine hippurate and methenamine mandelate; dosage expressed in terms of the salt.
Pediatric Patients
Prophylaxis or Suppression of Urinary Tract Infections (UTIs)
Methenamine Hippurate
Oral
Children <6 years of age: Dosage not established.
Children 6–12 years of age: 500 mg to 1 g twice daily (morning and night).
Children >12 years of age: 1 g twice daily (morning and night).
Methenamine Mandelate
Oral
Children <6 years of age: 18.4 mg/kg 4 times daily, (after meals and at bedtime).
Children 6–12 years of age: 500 mg 4 times daily (after meals and at bedtime).
Children >12 years of age: 1 g 4 times daily (after meals and at bedtime).
Alternatively, some clinicians recommend 50 mg/kg daily in 3 divided doses for children.
Adults
Prophylaxis or Suppression of Urinary Tract Infections (UTIs)
Methenamine Hippurate
Oral
1 g twice daily (morning and night).
Methenamine Mandelate
Oral
1 g 4 times daily (after meals and at bedtime).
Special Populations
No special population dosage recommendations at this time.
Cautions
Contraindications
Warnings/Precautions
Warnings
Dysuria
Large doses (8 g daily for 3–4 weeks) have caused bladder irritation, painful and frequent micturition, albuminuria, and gross hematuria.
Dysuria may be controlled by reducing dosage and/or reducing urine acidification.
Hepatic Effects
Patients with preexisting hepatic insufficiency may have adverse effects from the small amounts of ammonia and formaldehyde that are produced following administration of methenamine. Acute hepatic failure may occur in some patients.
Transient elevations in serum AST and ALT concentrations have occurred in patients receiving methenamine hippurate.
Perform periodic liver function tests in patients receiving methenamine hippurate, especially in those with hepatic impairment. (See Hepatic Impairment under Cautions.)
Sensitivity Reactions
Tartrazine Sensitivity
Hiprex® tablets contain tartrazine (FD&C yellow No. 5), which may cause allergic reactions, including bronchial asthma, in susceptible individuals. Incidence of tartrazine sensitivity is low, but it frequently occurs in patients who are sensitive to aspirin.
General Precautions
Acidic Urine
Ensure that urine is maintained at an acidic pH during treatment, especially when causative organisms are urea-splitting strains of Proteus or Pseudomonas. (See Dosage and Administration.)
Manufacturer of methenamine mandelate states the drug is not recommended if urine acidification is contraindicated or unattainable (e.g., when some urea-splitting bacteria are present).
Selection and Use of Anti-infectives
To reduce development of drug-resistant bacteria and maintain effectiveness of methenamine and other antibacterials, use only to prevent infections proven or strongly suspected to be caused by susceptible bacteria.
When selecting or modifying anti-infective therapy, use results of culture and in vitro susceptibility testing. In the absence of such data, consider local epidemiology and susceptibility patterns when selecting anti-infectives for empiric therapy.
Specific Populations
Pregnancy
Category C.
One manufacturer of methenamine hippurate states that safety during the last trimester is suggested, but not definitely proven.
Effects of methenamine during labor and delivery are unknown and there are no recognized uses for the drug during labor or delivery.
Lactation
Distributed into milk; discontinue nursing or the drug.
Pediatric Use
Has been used in children without unusual toxicity.
Geriatric Use
Insufficient experience in patients ≥65 years of age to determine whether they respond differently than younger adults; clinical experience has not identified differences.
Select dosage with caution, usually starting at the low end of the dosage range, because of possible age-related decreases in hepatic, renal, and/or cardiac function and concomitant disease and drug therapy.
Hepatic Impairment
Perform periodic liver function tests.
Methenamine hippurate is contraindicated in those with severe hepatic insufficiency.
Common Adverse Effects
GI disturbances (nausea, vomiting, diarrhea, abdominal cramps, anorexia), pruritus, rash, dysuria.
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