Methenamine (Hexamine) : Therapeutic uses, Dosage & Side Effects
Methenamine (Hexamine/Urotropine/ Formin/ Aminoform) is a hexamethylene-tetramine. This drug exerts no antimicrobial activity in blood and tissues, including kidney parenchyma. It is frequently formulated with a weak acid (for example, mandelic acid or hippuric acid) to keep the urine acidic. Antacids, such as sodium bicarbonate, should be avoided.
Dose : 0.5-1 g
Route : Oral
Onset of action : ?
Plasma Half-life : ?
Duration of action : ?
Bioavailability : ?
Plasma protein binding : ?
Metabolism : ?
Pregnancy risk category : ?
Chemical formula : C6H12N4
IUPAC name : 1,3,5,7-Tetraazatricyclo[188.8.131.52]decane
Trade name : Mandelamine, Hiprex, Urex
Antibacterial spectrum : Urea-splitting bacteria that alkalinize the urine, such as Proteus species, are usually resistant to the action of methenamine. It is primarily bacteriostatic, but may be cidal at higher concentrations and in acidic urine. The drug is generally active against E. coli, enterococci and staphylococci. Enterobacter aerogenes is generally resistant.
Therapeutic uses : Methenamine is primarily used for chronic suppressive therapy(chronic, resistant type of urinary tract infections, not involving kidney substance) to reduce the frequency of UTIs. It is not a good drug for acute urinary tract infections or for catheterization prophylaxis.
Routine use in patients with chronic urinary catheterization to reduce catheter associated bacteriuria or catheter-associated UTI is not generally recommended. It should not be used to treat upper UTIs (for example, pyelonephritis).
Mechanism of action : Methenamine is inactive as such. It decomposes slowly at an acidic pH of 5.5 or less (acidic) in urine to release formaldehyde which is toxic to bacteria & inhibits all bacteria. Acidic urine is essential for its action; urinary pH must be kept below 5.5 by administering some organic acid which is excreted as such, e.g. mandelic acid or ascorbic acid.
Resistance : Resistance to formaldehyde does not occur, but methenamine is not popular now .
Side effects : Methenamine may cause following side effects :
- Gastrointestinal distress.
- Gastritis (due to release of formaldehyde in stomach-patient compliance is often poor due to this).
- Chemical cystitis, haematuria & albuminuria may develop with high doses given for long periods.
- CNS symptoms are produced occasionally.
- Methenamine Χ Sulfonamides : ↑ risk of crystalluria and mutual antagonism. Sulfonamides, such as cotrimoxazole, react with formaldehyde.
- Large doses of Methenamine (8 grams daily for 3 to 4 weeks) have caused bladder irritation, painful and frequent micturition, albuminuria, and gross hematuria.
- Methenamine mandelate is contraindicated in patients with renal insufficiency or renal failure (mandelic acid accumulates in blood which cause acidosis).
- Contraindicated in patients with liver disease (the released NH3 is not detoxified).
- Contraindicated in patients with severe dehydration.
For detailed query or in case of uncertainty, Always consult your doctor or pharmacist.