Methenamine can be an effective urinary antiseptic agent for certain patients at risk for a urinary tract infection. Methenamine is metabolized to formaldehyde which suppresses bacteria provided it is present for long enough at a high enough urinary concentration.



(1) An acidified urine to <= pH 5-6 (with ascorbic acid 1-2 grams po q4h).

(2) Fluid restriction to concentrate the urine (target urinary specific gravity >= 1.015).

(3) A high enough dose to produce sufficient formaldehyde.



(1) Inability to sterilize the urine first.

(2) Hyperuricemia and/or gout. (risk of uric acid nephrolithiasis in acid urine with hyperuricuria)

(3) Hepatic insufficiency (due to generation of ammonia during metabolism to formaldehyde)

(4) Presence of an indwelling or condom-type urinary catheter (urine is drained off as it is formed).

(5) Concurrent therapy with a sulfonamide (precipitate on exposure to formaldehyde).

(6) Renal failure with creatinine clearance < 50 mL/min (for methenamine salts; pure methenamine can be used).


Do not use for treating:

(1) pyelonephritis


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