Adult: For prophylaxis and long-term suppression of chronic or recurrent cases: As methenamine hippurate: 1,000 mg bid. Dose may be increased to 1,000 mg tid in catheterised patients. As methenamine mandelate: 1,000 mg 4 times daily (after each meal and at bedtime). Treatment recommendations may vary among countries and between individual products (refer to specific product guidelines). Child: As methenamine hippurate: 6-12 years 500-1,000 mg bid; >12 years 1,000 mg bid. As methenamine mandelate: 6-12 years 500 mg 4 times daily; >12 years 1,000 mg 4 times daily (after each meal and at bedtime). Dosage and treatment recommendations may vary among countries and between individual products (refer to specific product guidelines).
What are the brands available for Methenamine in Hong Kong?
Antihydral
Renal Impairment
Contraindicated.
Hepatic Impairment
Contraindicated.
Contraindications
Hypersensitivity. Gout, metabolic acidosis, severe dehydration. Renal and hepatic impairment. Concomitant use with sulfonamides.
Special Precautions
Maintain an acidic urine pH, particularly during treatment of infections caused by urea-splitting organisms (e.g. Proteus, strains of Pseudomonas). Avoid or restrict alkalinising food and drugs; supplemental acidification may be instituted as necessary. Children. Pregnancy and lactation.
Adverse Reactions
Significant: Transient elevations in serum AST and ALT concentrations; bladder irritation, painful and frequent micturition, albuminuria, and gross haematuria occurred after administration of large doses (8,000 mg daily for 3-4 weeks). Gastrointestinal disorders: Nausea, vomiting, diarrhoea, abdominal pain. Skin and subcutaneous tissue disorders: Rash, pruritus.
Perform culture and susceptibility tests; consult local institutional recommendations before treatment initiation. Monitor LFTs (periodically) and urinalysis.
Overdosage
Symptoms: Nausea, vomiting, tinnitus, vertigo, metabolic acidosis, albuminuria and haematuria. Management: Symptomatic and supportive treatment. Large quantities of water and 2-3 teaspoonfuls of Na bicarbonate may be given to treat bladder symptoms.
Drug Interactions
Increased risk of crystalluria when used concomitantly with sulfonamides. Decreased efficacy with alkalinising agents (e.g. Na bicarbonate). Antacids can increase urine pH, which may reduce the effect of methenamine.
Lab Interference
May lead to falsely elevated results in the determination of urinary catecholamine and vanillylmandelic acid and falsely elevated 17-hydroxycorticosteroid levels when the Porter-Silber method is used. May cause falsely decreased levels of urine estriol when acid hydrolysis techniques are used and falsely decreased levels of 5-hydroxyindoleacetic acid when nitrosonaphthol methods are used.
Action
Description: Mechanism of Action: Methenamine is a synthetic urinary antibacterial agent with a wide spectrum of activity against organisms, including gram-positive and gram-negative bacteria. In an acidic medium (acidic urine), it is hydrolysed into formaldehyde, a non-specific antibacterial agent that usually has bactericidal action. Furthermore, the acid portions of methenamine salts (hippuric acid or mandelic acid) help maintain urine acidity and may also exhibit some antibacterial activity. Onset: 30 minutes. Pharmacokinetics: Absorption: Readily absorbed from the gastrointestinal tract. Time to peak plasma concentration: 1-2 hours. Distribution: Crosses the placenta; enters breast milk (small amounts). Volume of distribution: 0.6 L/kg. Metabolism: Hydrolysed to ammonia and formaldehyde in acidic urine; Max hydrolysis occurs in urine pH ≤5.5. Excretion: Via urine (approx 70-90% as unchanged drug). Elimination half-life: Approx 4 hours.
Chemical Structure
Methenamine Source: National Center for Biotechnology Information. PubChem Compound Summary for CID 4101, Methenamine. https://pubchem.ncbi.nlm.nih.gov/compound/Methenamine. Accessed Nov. 26, 2024.
J01XX05 - methenamine ; Belongs to the class of other antibacterials. Used in the systemic treatment of infections.
References
Anon. Methenamine. AHFS Clinical Drug Information [online]. Bethesda, MD. American Society of Health-System Pharmacists, Inc. https://www.ahfscdi.com. Accessed 06/11/2024.Brayfield A, Cadart C (eds). Methenamine. Martindale: The Complete Drug Reference [online]. London. Pharmaceutical Press. https://www.medicinescomplete.com. Accessed 06/11/2024.iNova Pharmaceuticals (New Zealand), Ltd. Hiprex 1 g Tablets data sheet 15 June 2023. Medsafe. http://www.medsafe.govt.nz. Accessed 06/11/2024.Joint Formulary Committee. Methenamine Hippurate. British National Formulary [online]. London. BMJ Group and Pharmaceutical Press. https://www.medicinescomplete.com. Accessed 06/11/2024.Methenamine Hippurate 1 g Tablets (Mercury Pharmaceuticals Ltd.). MHRA. https://products.mhra.gov.uk. Accessed 06/11/2024.Methenamine Hippurate Tablet (Jubilant Cadista Pharmaceuticals Inc.). DailyMed. Source: U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed. Accessed 06/11/2024.Methenamine Mandelate Tablet, Film Coated (Chartwell RX, LLC). DailyMed. Source: U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed. Accessed 06/11/2024.Methenamine. Gold Standard Drug Database in ClinicalKey [online]. Elsevier Inc. https://www.clinicalkey.com. Accessed 06/11/2024.Methenamine. UpToDate Lexidrug, Lexi-Drugs Multinational Online. Waltham, MA. UpToDate, Inc. https://online.lexi.com. Accessed 06/11/2024.
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