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

Trimethoprim

Brand names: Monotrim

Adult dose

Dose: 200mg twice daily × 7 days (UTI). Prophylaxis: 100mg once nightly
Route: Oral
Frequency: Twice daily (treatment) or once nightly (prophylaxis)
Uncomplicated UTI in women: 200mg BD × 3–7 days. Reduce dose in renal impairment: eGFR 15–30 → 50% dose reduction.

Clinical pearls

  • Creatinine rise of ~10% expected — tubular secretion inhibition, not AKI. Check potassium instead.
  • High rates of E. coli resistance (>30%) in many areas — check local sensitivities
  • Co-trimoxazole (trimethoprim + sulfamethoxazole) = Septrin — wider spectrum (PCP prophylaxis, Pneumocystis treatment)
  • Avoid in first trimester — folate antagonist (risk of neural tube defects)

Contraindications

  • Pregnancy (first trimester — folate antagonist)
  • Blood dyscrasias
  • Folate deficiency
  • Severe renal impairment

Side effects

  • Rash
  • Nausea
  • Hyperkalaemia (especially with ACE-I/ARB/spironolactone)
  • Megaloblastic anaemia (prolonged use)
  • Rises in serum creatinine (reduces tubular creatinine secretion — not true AKI)

Interactions

  • ACE inhibitors / ARBs / spironolactone — hyperkalaemia risk
  • Methotrexate — increased toxicity
  • Warfarin — increased INR
  • Phenytoin — toxicity

Monitoring

  • Renal function and serum potassium (especially with RAAS inhibitors)
  • U&E (prolonged use)

Reference: PHE Antibiotic Guidelines; NICE CKS UTI. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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