Methenamine

Aka: Methenamine, Methenamine Hippurate, Methenamine Mandelate

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II. Indications

  1. Recurrent Cystitis
    1. Short term prophylaxis in patients without renal tract abnormalities

III. Mechanism

  1. Metabolized to formaldehyde in the urine, killing local Microbes

IV. Dosing

  1. Methenamine Hippurate (preferred)
    1. Adult: 1 g orally twice daily
    2. Child age 6-12 years: 500 mg orally twice daily
  2. Methenamine Mandelate
    1. Older preparation with less evidence (released before 1938 FDA reviews started)
    2. Adult: 1 g orally four times daily
    3. Child age 6-12 years: 500 mg orally four times daily
    4. Child age <6 years: 250 mg orally four times daily

V. Drug Interactions

  1. Trimethoprim Sulfamethoxazole (TMP-SMZ)
    1. Formaldehyde (Methenamine metabolite) reacts with Sulfonamides, forming precipitates
    2. Results in Crystalluria

VI. Resources

  1. Methenamine Hippurate (DailyMed)
    1. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=9f002ad9-58e7-44f2-a980-3cc3f36bbd9b
  2. Methenamine Mandelate (DailyMed)
    1. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=cc299e8a-cf9a-4dc6-b6e6-972141e59296

VII. References

  1. (2026) Presc Ins 33(6): 1-3
  2. Weidner (2018) Email communication, received 9/1/2018
  3. Lee (2012) Cochrane Database Syst Rev (10): CD003265 +PMID: 23076896 [PubMed]

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