CautionTrimethoprim
Trimethoprim
Dihydrofolate reductase inhibitor
Avoid T1 (folate antagonist) — UTI option T2/T3.
Oral
First-line for
- Lower UTI outside T1 (with folate supplementation)
Mechanism
Inhibits bacterial DHFR → ↓ folate → ↓ DNA synthesis.
Trim trimester rule
- T1 — avoid (NTD)
- T2/3 — OK with folate
- Always folic acid 5 mg if used
Dosing
- UTI
- 200 mg PO BD ×3 days (non-pregnant) / 7 days (pregnant)
Contraindications
- AbsoluteT1 pregnancy without folate; folate deficiency, blood dyscrasia
Side effects
- Hyperkalaemia
- Rash, blood dyscrasia
Fetal & maternal notes
- • NTD risk in T1 — co-prescribe folic acid 5 mg.
Key interactions
Methotrexate
AvoidPancytopenia
ACEi/spironolactone
AvoidHyperkalaemia
Phenytoin
Caution↑ levels
Clinical pearls
- 💡 Resistance rising — check local antibiogram.
- 💡 Nitrofurantoin preferred T1.
Content last reviewed: REPLACE_ME (e.g. 2026-07)
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