Methenamine

Indications
 Oral
Treatment and prophylaxis of chronic, uncomplicated lower urinary-tract infections., Asymptomatic bacteriuria
Adult: As hippurate: 1 g bid, up to tid in catheterised patients. As mandelate: 1 g 4 times daily.
Child: <6 yr: as mandelate: 18 mg/kg four times daily. 6-12 yr: as hippurate: 0.5-1 g bid; as mandelate: 0.5 g four times daily.
Contraindications
 Renal insufficiency, hepatic insufficiency, severe dehydration.
Warnings / Precautions
 Metabolic acidosis, gout, pregnancy and lactation.
Adverse Reactions
 Nausea, vomiting, diarrhoea, dyspepsia, rash, irritation and inflammation of urinary tract especially bladder, painful and frequent micturition, proteinuria and haematuria.
Drug Interactions
 Effects diminished when combined with alkalinizing agents, antacids, potassium citrate and diuretics like acetazolamide and thiazides. Ascorbic acid or ammonium chloride increases acidification of urine and may increase methenamine efficacy.
Potentially Fatal: May precipitate crystalluria with sulphonamides, avoid concurrent use.
See Below for More methenamine Drug Interactions
Lab Interactions
 Interferes with urinary estimation of catecholamines, 17-hydroxycorticosteroids and oestrogen.
Food Interactions
 Restrict intake of alkalinising food.
Mechanism of Actions
 Methenamine is a synthetic urinary antibacterial taken as either the hippurate or mandelate salt. The antimicrobial activity of methenamine depends on its conversion to formaldehyde in acidic urine. It is used for treatment of urinary tract infections and is active against both gram-positive and gram-negative bacteria.
Absorption: Readily absorbed from the GI tract.
Distribution: Methenamine crosses the placenta and is distributed into milk.
Metabolism: Methenamine is excreted into the urine where it is hydrolysed to formaldehyde and ammonia (in acidic conditions only, maximum hydrolysis at <pH 5.5).
Excretion: Almost completely excreted in the urine.
Storage Conditions
 Oral: Store at 25°C.
ATC Classification
 J01XX05 - methenamine ; Belongs to the class of other antibacterials. Used in the systemic treatment of infections.
Storage
 Oral: Store at 25°C.
Available As
  • Methenamine 0.5 mg
  • Methenamine 1 gm
  • Methenamine 500 mg

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