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Antibiotic — Folate Synthesis Inhibitor Pregnancy: D (first trimester); C (second/third trimester)

Trimethoprim (UTI)

Brand names: Monotrim

Adult dose

Dose: 200 mg twice daily for 7 days (uncomplicated UTI); 100 mg at night (prophylaxis)
Route: oral
Frequency: twice daily (treatment) or once nightly (prophylaxis)
Max: 400 mg/day
Avoid in first trimester (folate antagonism); prophylaxis dose 100 mg at night; increasing resistance — check local sensitivities

Paediatric dose

Dose: 2 mg/kg
Route: oral
Frequency: twice daily
Max: 200 mg twice daily
Concentration: 50 mg/5 ml suspension mg/ml
1–5 months: 25 mg BD; 6 months–5 years: 50 mg BD; 6–11 years: 100 mg BD; >12 years: 200 mg BD

Dose adjustments

Renal

50% dose if eGFR 15–30 ml/min; avoid if eGFR <15 ml/min

Hepatic

No dose adjustment required in mild-moderate hepatic impairment

Paediatric weight-based calculator

1–5 months: 25 mg BD; 6 months–5 years: 50 mg BD; 6–11 years: 100 mg BD; >12 years: 200 mg BD

Clinical pearls

  • First-line for uncomplicated UTI per NICE NG112 only if local resistance <20%
  • Avoid in first trimester — folate antagonist; use nitrofurantoin instead
  • Hyperkalaemia mechanism: blocks ENaC channels like potassium-sparing diuretics

Contraindications

  • First trimester pregnancy
  • Blood dyscrasias
  • Severe renal impairment (eGFR <15)
  • Methotrexate (folate antagonism)

Side effects

  • Nausea
  • Vomiting
  • Rash
  • Pruritus
  • Megaloblastic anaemia (prolonged use)
  • Hyperkalaemia (at high doses — K-sparing effect)

Interactions

  • Methotrexate (increased toxicity)
  • Warfarin (enhanced anticoagulation)
  • ACE inhibitors/ARBs (hyperkalaemia)
  • Phenytoin (increased levels)

Monitoring

  • FBC (prolonged use)
  • Renal function
  • Serum potassium (high doses)

Reference: BNFc; BNF 86; NICE NG112; PHE antimicrobial guidelines. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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