Methenamine

Oral
Asymptomatic bacteriuria, Treatment and prophylaxis of chronic, uncomplicated lower urinary tract infections

Contraindicated.

Severe: Contraindicated.

Hypersensitivity. Gout, metabolic acidosis, severe dehydration. Renal and severe hepatic impairment. Concomitant use w/ sulfonamides or alkalinising agents.

Patient w/ indwelling urinary catheters; conditions wherein urine acidification is contraindicated or unattainable (e.g. presence of urea-splitting bacteria in the urine). Mild to moderate hepatic impairment. Childn. Pregnancy and lactation. Not intended for use in upper UTI.

Significant: Increased ALT/AST, dysuria.
GI: Dyspepsia, nausea, vomiting, diarrhoea.
Genitourinary: Bladder irritation, painful/frequent micturition, albuminuria, haematuria.
Dermatologic: Rash, pruritus.

PO: C

Monitor urinalysis, urine culture, LFT. Maintain an acidic urine, or give supplemental acidification (e.g. ammonium Cl, ascorbic acid, methionine), if necessary.

Symptoms: Vomiting, haematuria. Management: Immediately induce emesis or perform gastric lavage. Maintain adequate hydration, give copious quantities of water and 2-3 teaspoonfuls of Na bicarbonate.

Acetazolamide may inhibit conversion of methenamine into formaldehyde.
Potentially Fatal: Risk of crystalluria when given w/ sulfonamides (e.g. sulfamethizole) or urinary alkalinising agents (e.g. potassium citrate).

Decreased effect w/ alkalinising food.

May interfere w/ determination of urinary catecholamines and vanillylmandelic acid (VMA) using fluorometric procedures, and 17-hydroxycorticosteroid using Porter-Silber method, resulting in falsely high results. May falsely decrease 5-hydroxyindoleacetic acid (5HIAA) levels when using nitrosonaphthol methods, and estriol levels when using acid hydrolysis techniques.

Description: Methenamine is a urinary antibacterial agent. Its action depends on its conversion in acidic urine into formaldehyde, a nonspecific bactericidal agent, that is active against both gram+ve and gram-ve bacteria
Onset: W/in 30 min (as hippurate).
Pharmacokinetics:
Absorption: Readily absorbed from the GI tract. Time to peak plasma concentration: 1-2 hr.
Distribution: Widely distributed in the body. Crosses the placenta and enters breast milk. Volume of distribution: 0.6 L/kg.
Metabolism: Undergoes hydrolysis in the urine at pH ≤5.5 into ammonia and formaldehyde. Approx 10-25% is metabolised in the liver.
Excretion: Via urine (approx 70-90%, as unchanged drug). Elimination half-life: Approx 4 hr.

Source: National Center for Biotechnology Information. PubChem Database. Methenamine, CID=4101, https://pubchem.ncbi.nlm.nih.gov/compound/Methenamine (accessed on Jan. 22, 2020)

Store between 15-30°C.

Urinary Antiseptics

J01XX05 - methenamine ; Belongs to the class of other antibacterials. Used in the systemic treatment of infections.

Anon. Methenamine. Lexicomp Online. Hudson, Ohio. Wolters Kluwer Clinical Drug Information, Inc. https://online.lexi.com. Accessed 28/07/2017.

Buckingham R (ed). Methenamine. Martindale: The Complete Drug Reference [online]. London. Pharmaceutical Press. https://www.medicinescomplete.com. Accessed 27/07/201.

Joint Formulary Committee. Methenamine Hippurate. British National Formulary [online]. London. BMJ Group and Pharmaceutical Press. https://www.medicinescomplete.com. Accessed 27/07/2017 .

McEvoy GK, Snow EK, Miller J et al (eds). Methenamine, Methenamine Hippurate, Methenamine Mandelate. AHFS Drug Information (AHFS DI) [online]. American Society of Health-System Pharmacists (ASHP). https://www.medicinescomplete.com. Accessed 27/07/2017 .

Methenamine Hippurate Tablets, USP 1 g (CorePharma, LLC). DailyMed. Source: U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed/. Accessed 28/07/2017.

Methenamine Mandelate USP (Edenbridge Pharmaceuticals, LLC). DailyMed. Source: U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed/. Accessed 28/07/2017.