Methenamine and Methenamine Salts

 Action In acidic urine, methenamine is hydrolyzed to ammonia and formaldehyde, which is bactericidal to certain bacteria in urine. Acid salts (methenamine mandelate and hippurate) have some nonspecific bacteriostatic activity and help to maintain low urine pH.

 Indications Suppression or elimination of bacteriuria associated with pyelonephritis, cystitis and other chronic urinary tract infections; treatment of infected residual urine, sometimes accompanying neurologic disease or diabetes.

 Contraindications Renal insufficiency; severe dehydration; severe hepatic insufficiency with hyperammonemia; acute urinary tract infections involving renal parenchyma.

 Route/Dosage

Methenamine Hippurate

ADULTS & CHILDREN > 12 YR: PO 1 g bid. CHILDREN 6–12 YR: PO 500 mg-1 g bid.

Methenamine Mandelate

ADULTS: PO 1 g qid after meals and at bedtime. CHILDREN 6–12 YR: PO 500 mg qid. CHILDREN < 6 YR: PO 250 mg for q 30 lb body weight qid (18.4 mg/kg qid).

 Interactions

Sulfonamides: May increase chance of crystalluria. Urine alkalizers (acetazolamide, sodium bicarbonate or carbonate): Prevents hydrolysis of methenamine to formaldehyde with possible decrease in antimicrobial action.

 Lab Test Interferences Methenamine may interfere with laboratory urine determinations of 17-hydroxycorticosteroids, catecholamines and vanillylmandelic acid (false increases) and 5-hydroxyindoleacetic acid (false decrease). Taken during pregnancy, can interfere with laboratory tests for urine estriol (false decrease) when acid hydrolysis procedure is used; use enzymatic hydrolysis procedure.

 Adverse Reactions

CNS: Headache. DERM: Pruritus; urticaria; erythematous eruptions; rash. GI: Nausea; vomiting; cramps; diarrhea; stomatitis; anorexia. GU: Bladder irritation; dysuria; proteinuria; hematuria; frequency; urgency; rystalluria. HEMA: Serum transaminase elevation. RESP: Dyspnea. OTHER: Generalized edema.

 Precautions

Pregnancy: Category C. Lactation: Undetermined. Debilitated patients and patients with swallowing difficulty: Use with caution to avoid inducing lipoid pneumonia. Acid urine: If acidification of urine cannot be obtained or is contraindicated, drug is not recommended. Gout: May cause precipitation of urate crystals in urine. Lipoid pneumonia: Methenamine mandelate oral suspension is vegetable oil–based; spiration could result in lipoid pneumonitis. Tartrazine sensitivity: Some products contain tartrazine, which may cause rash or bronchial asthma in susceptible patients.

 Administration/Storage

• Administer after meals and at bedtime to minimize GI distress.
• Reconstitute granules by dissolving 1 packet (500 mg-1 g) in 60–120 ml of water immediately before use. Solution may be cloudy.
• Urinary acidification using ascorbic acid to maintain low pH may be necessary.
• Store at room temperature in tightly closed container. Protect from excessive heat.

 Assessment/Interventions

• Obtain patient history, including drug history and any known allergies.
• Obtain clean-catch urine specimen for culture and sensitivity before beginning therapy.
• Monitor I&O, and watch for bladder irritation (painful/frequent urination, proteinuria, hematuria); dose may need to be decreased if these symptoms occur. Fluid intake should be maintained at 1500–2000 ml/day (if medically acceptable).
• Monitor liver function test results for transient increase in enzymes.
• Avoid concurrent use of sulfonamides (may cause precipitates), or any drugs that will alkalize urine.

 Patient/Family Education

• Explain significance of adequate hydration.
• Tell patient to report these symptoms to physician: painful urination, skin rash, headache, swelling or severe stomach upset.
• Instruct patient to avoid use of milk products and antacids while taking drug to help keep urine acidic and allow drug to work better. Instruct patient to take vitamin C and drink cranberry or prune juice to acidify urine.
• Caution patient not to self-medicate with otc medications containing sodium bicarbonate or sodium carbonate.
• Teach patient how to read dipstick tests for urine pH and specific gravity and to report to physician if required values are not attained.