Methenamine
Methenamine (also known as hexamine) is a urinary tract antiseptic used primarily for the prophylaxis and long-term management of recurrent lower urinary tract infections. **Clinical Pearl:** It is not a traditional antibiotic but acts as a prodrug. Because its efficacy relies entirely on its conversion to formaldehyde in an acidic environment, it is ineffective in alkaline urine or for systemic infections. Bacterial resistance to formaldehyde does not naturally occur, making it useful for chronic management.
Mechanism: Methenamine is a prodrug that is hydrolyzed in an acidic environment (urine pH < 5.5) → **ammonia** and **formaldehyde**. Formaldehyde acts as a non-specific bactericidal agent that denatures bacterial proteins and nucleic acids. It requires sufficient time in the bladder to convert and exert its effects.
Routes of administration
Contraindications
- Renal insufficiency (can exacerbate systemic acidosis)
- Severe hepatic disease (due to ammonia production)
- Severe dehydration
- Metabolic acidosis
- Concurrent use of sulfonamides
Adverse effects
- Gastrointestinal upset (nausea, vomiting, diarrhea)
- Dysuria or pollakiuria (due to formaldehyde irritation)
- Hematuria
- Crystalluria (at high doses)
Drug interactions
- Sulfonamides · Formaldehyde forms an insoluble precipitate with sulfonamides, increasing the risk of crystalluria and renal damage. · major
- Urinary alkalinizers (e.g., Potassium citrate, Sodium bicarbonate) · Raises urine pH, preventing the conversion of methenamine to its active form (formaldehyde). · major
- Thiazide diuretics · May increase urine pH and promote diuresis, reducing formaldehyde concentration and contact time in the bladder. · moderate
Monitoring
- Urine pH (target < 5.5)
- Urinalysis (monitor for crystalluria or hematuria)
- Renal function
- Hepatic function (in prolonged use)
Overdose
Overdosage primarily manifests as severe gastrointestinal distress (vomiting, diarrhea) and urinary tract irritation (hematuria, dysuria, crystalluria). Treatment is supportive and symptomatic. Alkalinizing the urine can halt further conversion to formaldehyde and reduce urinary irritation.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.