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Urinary antiseptic Pregnancy: There is inadequate evidence of safety of methenamine hippurate in human pregnancy, but it has been in wide use for many years without apparent ill consequence; animal studies are insufficient with respect to reproductive toxicity. As a precautionary measure, it is preferable to avoid use during pregnancy. Breast-feeding: methenamine is excreted in breast milk but the quantities will be insignificant to the infant, so mothers can breast feed their infants.

Methenamine hippurate

Brand names: Hiprex

Methenamine hippurate is a urinary antiseptic used for prophylaxis against recurrent lower urinary tract infection rather than for treating acute infection.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 1 g twice daily
Route: Oral
Frequency: Twice daily
Max: 1 g three times daily — in patients with catheters the dosage may be increased to 1 g three times daily
Older people: no special dosage recommendations. Paediatric (non-weight-based, so not captured as a per-kg dose): children under 6 years — not recommended; children 6–12 years — 500 mg twice daily. Administration: the tablets may be halved, or they can be crushed and taken with a drink of milk or fruit juice if the patient prefers. Source: UK SPC for Hiprex 1 g Tablets (eMC product 1530). Cross-check only (US labelling, not UK): 1 tablet (1.0 g) twice daily, morning and night, for adults and paediatric patients over 12 years; 1/2 to 1 tablet (0.5 to 1.0 g) twice daily for paediatric patients 6 to 12 years; since antibacterial activity is greater in acid urine, restriction of alkalinising foods and medications is desirable.

Dose adjustments

Renal

Contraindicated in severe renal failure (creatinine clearance or GFR <10 mL/min) and in renal parenchymal infection. May be used where mild (20–50 mL/min) to moderate (10–20 mL/min) renal insufficiency is present. If the GFR is not available the serum creatinine concentration can be used as a guide. Also contraindicated in hepatic dysfunction.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

  • Hypersensitivity to the active substance or to any of the excipients
  • Hepatic dysfunction
  • Renal parenchymal infection
  • Severe dehydration
  • Metabolic acidosis
  • Severe renal failure (creatinine clearance or GFR <10 mL/min)
  • Gout
  • Concurrent administration with sulphonamides (possibility of crystalluria) or with alkalising agents such as a mixture of potassium citrate

Side effects

  • Gastric irritation (uncommon)
  • Irritation of the bladder (uncommon)
  • Nausea and vomiting (uncommon)
  • Rash and pruritus (uncommon)
  • Diarrhoea and abdominal pain (frequency not known)

Interactions

  • Sulphonamides — should not be given concurrently because of the possibility of crystalluria
  • Alkalising agents such as potassium citrate — should not be given concurrently
  • Acetazolamide — concurrent use should be avoided as the desired effect of hexamine will be lost
  • Laboratory interference — depending on the analysing method used, methenamine can affect the determination of steroids, catecholamines and 5-hydroxyindole acetic acid from urine and give false results

Clinical monograph

How it works

In acidic urine it is hydrolysed to formaldehyde, which exerts a non-specific antibacterial effect within the urinary tract.

Prescribing in practice

  • It is for prophylaxis only and is not effective for treating an established acute urinary tract infection or upper urinary tract infection.
  • Efficacy depends on acidic urine and adequate urinary formaldehyde, so it is unsuitable where urine is persistently alkaline or where there is significant urinary stasis.
  • Avoid in significant renal or hepatic impairment, dehydration and metabolic disorders, and do not combine with sulfonamides, as advised in the SPC.

Monitoring

Monitoring is largely clinical, reviewing the frequency of breakthrough infections and tolerability rather than routine laboratory tests.

Counselling the patient

  • This medicine helps prevent infections coming back; it is not a treatment for an active infection.
  • Maintain a good fluid intake.
  • Report bladder discomfort, rash or if infections keep occurring despite treatment.

Evidence & guidelines

The ALTAR trial supported methenamine hippurate as a non-antibiotic option for preventing recurrent urinary tract infection in women.

Reference: NICE NG112; ALTAR trial; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

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