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Antifolate Antibiotic — Urinary (Obstetric Caution) Pregnancy: Avoid in first trimester and near term; use with caution in second/third trimester

Trimethoprim (UTI in Pregnancy — Caution)

Brand names: Monotrim

Adult dose

Dose: 200 mg twice daily for 7 days (UTI treatment); 100 mg at night (prophylaxis)
Route: Oral
Frequency: Twice daily (treatment); once nightly (prophylaxis)
Max: 400 mg/day
AVOID in first trimester — folate antagonist, risk of neural tube defects. Use in second/third trimester only when other agents unsuitable. Avoid at term — neonatal methaemoglobinaemia risk

Paediatric dose

Dose: Not applicable in this obstetric context N/A/kg
Route: Oral
Frequency: N/A
Max: N/A
Maternal medication

Dose adjustments

Renal

Avoid if eGFR under 15; reduce dose if eGFR 15-30

Hepatic

Use with caution in severe hepatic impairment

Paediatric weight-based calculator

Maternal medication

Clinical pearls

  • AVOID in first trimester: trimethoprim inhibits dihydrofolate reductase — folate antagonism during organogenesis increases neural tube defect risk; use cefalexin or nitrofurantoin instead
  • Second and third trimester: acceptable if other agents unsuitable and urine culture confirms susceptibility; avoid from 36 weeks (neonatal methaemoglobinaemia)
  • Asymptomatic bacteriuria in pregnancy: treat regardless of symptoms — increases risk of pyelonephritis, preterm birth, and low birth weight if left untreated
  • Hyperkalaemia: trimethoprim blocks the epithelial sodium channel in collecting duct (amiloride-like effect) — clinically significant with ACEi, ARBs, or potassium-sparing diuretics; monitor K
  • NICE NG109: trimethoprim is listed as a first-line UTI option in non-pregnant adults but specifically avoided in first trimester of pregnancy

Contraindications

  • First trimester of pregnancy
  • Near term (neonatal methaemoglobinaemia risk)
  • Megaloblastic anaemia due to folate deficiency

Side effects

  • Nausea and GI upset
  • Rash
  • Folate deficiency (prolonged use)
  • Hyperkalaemia (blocks renal K excretion)
  • Bone marrow suppression (rare)

Interactions

  • Methotrexate (additive antifolate toxicity — dangerous)
  • Warfarin (increases INR)
  • Potassium-sparing diuretics / ACEi (hyperkalaemia)
  • Phenytoin (increased levels)

Monitoring

  • Urine culture (sensitivity confirmation)
  • Test of cure (7 days post-treatment)
  • Electrolytes (K) if at risk

Reference: BNFc; BNF 90; NICE NG109 (UTI in Adults); NICE NG25 (Antenatal Care); RCOG Antenatal Care. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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