Drug Interaction:
Interacting drugs - summary
+ Methenamine
Sulfonamides- an insoluble precipiate between the sulfonamide and fomaldehyde may form in the urine
Urinary alkalinizers- alkalinizing agents amy decrease the efficay of metenamine by inhibting its conversiion to formaldehyde
Effects diminished when combined with alkalising agents, antacids, pot citrate, diuretics like acetozolamide and sulphonamides. Ascorbic acid or ammonium chloride increase acidification of urine. May precipitate(crysalluria) with sulphonamides.
Drug/Lab test interactions- Methenamine may interfere with laboratory urine determinations of 17-hydroxycorticosteroids, cathecholamines and vanillylmandelic acid (false increases) and 5-hydroxyindoleacetic acid (false decrease)
Methenamine taken during prergnancy can interfere with laboratory tests of urine estrol (resulting in unmeasurably low values ) when an acid hydrolysis procedure is used. This is due to the presence in the urine of methenamine or formaldehyde. Use enzymatic hydrolysis instead of acid hydrolysis.
Adverse Reaction:
Overall incidence- approximately 1 to 7%.
GI- nausea, vomiting, cramps, stomatitis, anorexia
GU- bladder irritationn dysuria, proteinuria, hematuria, urinary frequency/urgency, crystalluria (large doses)
Dermatologic- prutitus (rare), urticaria, erythemattous eruptions, rash
Miscellaneous- headache, dyspnea, lipoid pneuumonitis, generalized edema ( Rare)
Contra-Indications:
Renal insufficiency, hepatic insufficiency, hypersens.
Special precautions:
Dehydration, metabolic acidosis, gout.
Large doses- (8g daily for 3 to 4 weeks ) have caused bladder irritation, painful and frequent micturation, proteinuria and gross hematuria
Acid urine pH- should be maintained , especially when treating infections due to urea splitting organisms such as Proteus and strains of
Pseudomoinas. When acidification is contraindicated or unattianable (as with urea-splitting bacteria) the drug is not recommended.
Serum transaminases - have elevated mildly during treatment in a few instances and returned to normal while patients are still receiving methenamine hippurate. Perform liver function tests periodically on patients receiving methenamine hippurate, especially those with liver dysfunction.
Gout- methenamine salts may cause precipitation of urate crystals in the urine.
Tartrazine sewnsitivity- some of these products contain tartrazine, which may cause allergic-type reactions including bronchial asthma.
Warnings-
Pregnancy- give to pregnant women only if clearly needed.
Lactation- no adverse effects on the nursing infant have been reported.
Dosages/ Overdosage Etc:
Indications:
Urinary anti-infective
Dosage:
Adults- 1g 4 times a day, after meals and bed time
Children-(less than 6 years)- 0.25g/30lb(14kg) 4 times daily
Overdosage-
Immediately after ingestion of an overdose, further absorption of the drug may be miniomized by inducing vomiting or by gastric lavage, followed by administration of activated charcoal.
Force fluid, either oral or parentral to tolerance.
Missed dose-
1. If you miss a dose of this medicine, take it as soon as possible.
2. However, if it is almost time for next dose, skip the missed dose and go back to your regular dosing schedule.
3. Do not double doses.
Patient Information:
1. It may necessary to attempt to acidify the urine (eg, ascorbic acid, cranaberry juice)
2. Take with food to minimize GI upset.
3. Drink sufficient fluids to ensure adequate urine flow.
4. Avoid excessive intake of alkalinizing foods (milk products) or medication (biacrbonate, acetozolamide)
5. Notify physician if skin rash, painful urination or intolerable GI upset occurs.
Pharmacology/ Pharmacokinetics:
Pharmacology-
In acid urine methenamine is hydrolysed to ammonia and formaldehyde which is bacterial. Methenamine does not liberate formaldehyde in the serum. the acid salts (mandalelate and hippurate ) help maintain alow urine pH.
Pharmacokinetics-
Methenamine is readilt absorbed following oral administration . Approximately, 10% to 30% of methenamine is metabolised in the liver and has a half-life of 3 to 6 hours.
Interaction with Food:
Take with food to minimize GI upset.
Pregnancy and lactation:
Pregnancy-
Give to pregnant women only if claerly needed.
Lactation-
No adverse effects on the nursing infant have been reported.
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