Methenamine

Methenamine, also known as hexamethylenetetramine (HMTA), is a heterocyclic amine used as a urinary tract antiseptic. It is a prodrug that decomposes in acidic urine to yield formaldehyde and ammonia; the released formaldehyde is bactericidal within the urinary tract. Because hydrolysis requires an acidic environment, its activity depends on urine pH and is typically effective when the urine is acidified (approximately pH below 5.5).

Formulations include methenamine salts such as methenamine mandelate and methenamine hippurate, which improve solubility and tolerability. It is not an antibiotic and is not expected to treat established systemic infections; its primary use is prophylaxis of recurrent lower urinary tract infections in adults and, in some cases, in children, particularly when long-term suppression or avoidance of systemic antibiotics is desired.

Common adverse effects are usually mild and include gastrointestinal upset; hypersensitivity is rare. Cautions include use in patients with significant renal impairment or urinary tract obstruction, since excretion and activity rely on kidney function and urine acidity. It should be used with caution in patients who cannot maintain an adequately acidic urine, and monitoring of renal function and urinary pH is advisable. Contraindications include known hypersensitivity to methenamine.