Methenamine and Methenamine Salts
Pronouncation: (meh-THEN-uh-meen and meh-THEN-uh-meen salts)Class: Anti-infective agent Methenamine Hippurate
Trade Names:
Hiprex
- Tablets 1 g
Trade Names:
Urex
- Tablets 1 g
Trade Names:
Mandameth
- Tablets 0.5 mg
- Tablets, enteric-coated 1 g
Trade Names:
Mandelamine
- Tablets 0.5 g
- Tablets 1 g
Pharmacology
Feedback for Methenamine and Methenamine Salts
Compare with other drugs.
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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.
Pharmacokinetics
Absorption
Methenamine absorption is rapid (30% to 60% hydrolyzed by gastric acid if not enteric-coated). T max of urinary formaldehyde concentrations are 0.5 to 1.5 h for methenamine, 2 h for methenamine hippurate, and 3 to 8 h for methenamine mandalate.
Distribution
Methenamine is freely distributed to body tissues and fluids. Vd is approximately 0.56 L/kg.
Metabolism
Methenamine is metabolized in the liver (approximately 10 to 25%) and through hydrolysis in acidic urine (pH at least 5.5) to ammonia and formaldehyde.
Elimination
Methenamine is eliminated renally via glomerular filtration and tubular secretion. The t ½ is 3 to 6 h.
Indications and Usage
Suppression or elimination of bacteriuria associated with pyelonephritis, cystitis and other chronic UTIs; treatment of infected residual urine, sometimes accompanying neurologic disease or diabetes.
Contraindications
Renal insufficiency; severe dehydration; severe hepatic insufficiency with hyperammonemia; acute UTIs involving renal parenchyma.
Dosage and Administration
Methenamine HippurateAdults and Children (older than 12ߙyr of age)
PO 1 g twice daily.
Children (6 to 12 yr of age)PO 500 mg to 1 g twice daily.
Methenamine MandelateAdults
PO 1 g 4 times daily after meals and at bedtime.
Children (6 to 12 yr)PO 500 mg 4 times daily.
Children (younger than 6 yr)PO 250ߙmg for q 30 lb body weight 4 times daily (18.4 mg/kg 4 times daily).
Storage/Stability
Store at room temperature (59° to 86°F) in tightly closed container. Protect from excessive heat.
Drug Interactions
SulfonamidesMay increase chance of crystalluria.
Urine alkalizers (acetazolamide, sodium bicarbonate, carbonate)Prevents hydrolysis of methenamine to formaldehyde with possible decrease in antimicrobial action.
Laboratory Test Interactions
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.
Dermatologic
Pruritus; urticaria; erythematous eruptions; rash.
GI
Nausea; vomiting; cramps; diarrhea; stomatitis; anorexia.
Genitourinary
Bladder irritation; dysuria; proteinuria; hematuria; frequency; urgency; crystalluria.
Hematologic
Serum transaminase elevation.
Respiratory
Dyspnea.
Miscellaneous
Generalized edema.
Precautions
Pregnancy
Category C .
Lactation
Undetermined.
Special Risk Patients
Use with caution to avoid inducing lipoid pneumonia in debilitated patients and patients with swallowing difficulty.
Tartrazine Sensitivity
Some products contain tartrazine, which may cause rash or bronchial asthma in susceptible patients.
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; aspiration could result in lipoid pneumonitis.
Patient Information
- Explain significance of adequate hydration.
- Tell patient to report the following symptoms to health care provider: 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 health care provider if required values are not attained.
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