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Dihydrofolate reductase inhibitor / antibiotic Pregnancy: Avoid in first trimester (folate antagonist — NTD risk); caution in third trimester.

Trimethoprim

Brand names: Trimethoprim, Monotrim

Adult dose

Dose: 200 mg twice daily for 7 days (UTI); Prophylaxis: 100 mg at night
Route: Oral
Frequency: Twice daily (treatment); Once daily at night (prophylaxis)
Max: 400 mg/day
Uncomplicated UTI (women): 200 mg BD for 3–7 days. Prophylaxis (recurrent UTI): 100 mg at night. Pneumocystis prophylaxis (alternative): trimethoprim alone or co-trimoxazole. Avoid in first trimester (antifolate).

Paediatric dose

Dose: 4 mg/kg
Route: Oral
Frequency: Twice daily
Max: 200 mg/dose
Concentration: 50 mg/ml
1 month–5 years: 4 mg/kg (max 200 mg) BD. 6–11 years: 100 mg BD. 12–17 years: 200 mg BD. UTI prophylaxis: 2 mg/kg at night (max 100 mg).

Dose adjustments

Renal

eGFR 15–30: use normal dose for 3 days only; eGFR <15: avoid (if necessary, halve dose with monitoring).

Hepatic

Use with caution in severe hepatic impairment.

Paediatric weight-based calculator

1 month–5 years: 4 mg/kg (max 200 mg) BD. 6–11 years: 100 mg BD. 12–17 years: 200 mg BD. UTI prophylaxis: 2 mg/kg at night (max 100 mg).

Clinical pearls

  • First-line for uncomplicated lower UTI in UK (NICE NG112)
  • Causes elevation of serum creatinine without changing GFR — check potassium not creatinine trend
  • Significant hyperkalaemia risk with ACE inhibitors/ARBs/NSAIDs especially in elderly
  • Resistance variable (>20% E. coli resistant in some areas) — use MSU results where possible
  • Avoid in first trimester; nitrofurantoin preferred in second trimester

Contraindications

  • Blood dyscrasias
  • Severe renal impairment
  • First trimester of pregnancy (teratogenic — dihydrofolate reductase inhibitor)

Side effects

  • GI upset (nausea, vomiting)
  • Rash and pruritus
  • Hyperkalaemia (especially in renal impairment or with ACE inhibitors/ARBs)
  • Megaloblastic anaemia (prolonged use)
  • Elevation of serum creatinine (inhibits tubular secretion of creatinine — not true renal impairment)

Interactions

  • ACE inhibitors/ARBs — hyperkalaemia risk (trimethoprim has amiloride-like potassium-sparing effect)
  • Warfarin — increased anticoagulant effect
  • Phenytoin — increased phenytoin levels
  • Methotrexate — reduced folate-dependent effects; increased toxicity
  • Dofetilide — contraindicated (QT prolongation)

Monitoring

  • U&E (potassium — hyperkalaemia risk)
  • Renal function
  • FBC (prolonged use)

Reference: BNFc; BNF; NICE NG112 UTI; PHE Antimicrobial Prescribing 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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