Methenamine is indicated for prophylactic or suppressive treatment of frequently recurring urinary tract infections when long-term therapy is considered necessary. This drug should be used only after eradication of active infection with appropriate antimicrobial agents. To reduce the development of drug-resistant bacteria and maintain the effectiveness of Methenamine and other antibacterial drugs, it should be used only for infections that are proven or strongly suspected to be caused by susceptible bacteria. When culture and susceptibility data are available, they should be used to guide therapy selection or modification. In the absence of such data, local epidemiology and resistance patterns may guide empirical treatment.

Methenamine Hippurate has antibacterial activity because the methenamine component is hydrolyzed to formaldehyde in acidic urine. The hippuric acid component also has mild antibacterial activity and helps maintain urinary acidity. The drug is generally active against E. coli, enterococci, and staphylococci, while Enterobacter aerogenes is usually resistant. Urine must remain sufficiently acidic to inhibit urease-producing organisms such as Proteus and Pseudomonas.

Pharmacologically, within 30 minutes after ingestion of a single 1 g dose of Methenamine, antibacterial activity can be detected in urine. Continuous antibacterial activity is maintained when administered at the recommended dose of 1 g twice daily. More than 90% of the methenamine component is excreted in urine within 24 hours after a single dose. The hippurate component is also rapidly absorbed and excreted, reaching urine through both tubular secretion and glomerular filtration. This pathway may be particularly important in elderly patients or those with mild renal impairment.

For adults and pediatric patients over 12 years of age: 1 tablet twice daily (morning and night).

For pediatric patients 6 to 12 years of age: ½ to 1 tablet twice daily (morning and night).

Since the antibacterial activity of Methenamine is greater in acid urine, restriction of alkalinizing foods and medications is desirable. If necessary, as indicated by urinary pH and clinical response, supplemental acidification of the urine should be instituted. The efficacy of therapy should be monitored by repeated urine cultures.

Methenamine Hippurate tablets USP are contraindicated in patients with renal insufficiency, severe hepatic insufficiency, or severe dehydration. Methenamine preparations should not be administered with sulfonamides, as some sulfonamides may form an insoluble precipitate with formaldehyde in urine.

The safety of Methenamine in early pregnancy has not been established. In the last trimester, its safety is suggested but not conclusively proven. No adverse effects on the fetus were observed in studies conducted in pregnant rats and rabbits. Methenamine use during pregnancy may interfere with laboratory measurement of urinary estriol, leading to falsely low values when acid hydrolysis methods are used. This occurs due to the presence of methenamine and/or formaldehyde in urine. Enzymatic hydrolysis instead of acid hydrolysis can avoid this interference.

Use of Methenamine without a confirmed or strongly suspected bacterial infection or prophylactic indication is unlikely to benefit the patient and may increase the risk of antibiotic-resistant bacteria development.

An acidic urinary pH should be maintained during therapy, particularly in infections caused by urease-producing organisms such as Proteus and Pseudomonas.

In some cases, mild elevation of serum transaminase levels has been observed during treatment, which returned to normal even while therapy continued. Therefore, periodic liver function tests are recommended, especially in patients with existing liver dysfunction.

Geriatric Use: Clinical studies did not include sufficient numbers of patients aged 65 years or older to determine differences in response compared to younger patients. However, no significant differences have been reported in clinical experience. Dose selection in elderly patients should be cautious, usually starting at the lower end of the dosing range, due to more frequent reduced hepatic, renal, or cardiac function and concomitant diseases or medications.

Renal insufficiency and severe hepatic insufficiency: Methenamine is contraindicated in patients with renal impairment and severe liver disease.

Other genito-urinary preparations

Store below 30°C, protect from light and moisture. Keep out of reach of children.