Pharmacology · Urinary antiseptics
Methenamine !
A urinary antiseptic that turns into formaldehyde in acid urine. Used long-term to prevent recurrent cystitis; it does not breed resistance.
Updated 2026-11-01
Also called: hexamine, methenamine hippurate, methenamine mandelate
Mechanism of action
Methenamine itself is inactive. In acid urine it is hydrolysed to formaldehyde, which denatures bacterial proteins and nucleic acids. The hippurate salt also helps keep the urine acid. Because formaldehyde acts non-specifically, bacteria do not develop resistance to it.
- Absorbed and excreted into the urine
- Acid urine hydrolyses it to formaldehyde
- Formaldehyde denatures bacterial proteins
- Bacteria cannot establish in the bladder
Pharmacokinetics
- Absorption
- Absorbed by mouth.
- Elimination
- Urine, where it is converted to formaldehyde if the urine is acid.
Indications
- Prevention or suppression of frequently recurring urinary tract infections when long-term therapy is needed Urinary tract infection (acute cystitis) →
Adverse effects
Common
- Nausea and stomach upset
- Rash
- Painful or frequent urination with very high doses
Contraindications
- Kidney insufficiency
- Severe liver insufficiency
- Severe dehydration
Key interactions
- Sulfonamides
- Some form an insoluble precipitate with formaldehyde in urine; not combined.
- Urine alkalinisers (e.g. sodium bicarbonate, acetazolamide) and alkalinising foods
- Reduce its activity.
What is monitored
- Urine pH if infections recur
- Liver tests in long courses
Administration and safety
- Not used to treat an active infection: the infection is first cleared with another antibiotic
- Avoid antacids and urine-alkalinising products
Nursing care
- Explain that it is a preventive, long-term medicine
- Teach to report painful urination or blood in the urine
- Ask about sulfa drugs on the medication list
Pregnancy and breastfeeding
Its safe use in early pregnancy is not established; animal studies showed no fetal harm.
Test yourself
Think of the answer, then tap the question to see it.
What does methenamine need to work?
Acid urine, where it is hydrolysed to formaldehyde.
Why is it not combined with sulfonamides?
Some sulfonamides form an insoluble precipitate with formaldehyde in the urine.
Is methenamine a treatment for active cystitis?
No; it is used to prevent recurrences.
Related drugs
Related conditions
← Back to the class: Urinary antiseptics
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - METHENAMINE HIPPURATE tablet (DailyMed (U.S. National Library of Medicine), 2026); Methenamine: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).
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Representative model, for educational purposes only.