ClinCalc Pro
Menu
Calculators Drugs Pathways AI Dx Donate Get the App
Antibiotic (Sulfonamide + Folate Inhibitor) Pregnancy: D — folate antagonist, avoid especially in first trimester

Co-Trimoxazole

Brand names: Septrin

Adult dose

Dose: Treatment: 960mg (800/160mg) twice daily. PCP treatment: 60–120mg/kg/day in divided doses × 21 days. PCP prophylaxis: 960mg OD or 480mg OD
Route: Oral or IV
Frequency: Twice daily (standard); once daily (prophylaxis)
Each 960mg tablet = 800mg sulfamethoxazole + 160mg trimethoprim.

Paediatric dose

Dose: 6 mg/kg
Route: Oral or IV
Frequency: Twice daily
Max: 960mg per dose
Paediatric suspension: 240mg/5mL (200/40mg per 5mL). Standard: 6mg/kg TMP twice daily. PCP prophylaxis: HIV-exposed infants starting at 6 weeks.
Paediatric weight-based calculator

Paediatric suspension: 240mg/5mL (200/40mg per 5mL). Standard: 6mg/kg TMP twice daily. PCP prophylaxis: HIV-exposed infants starting at 6 weeks.

Clinical pearls

  • STOP immediately if blistering rash or mucosal involvement — SJS risk (especially in HIV patients)
  • First-line for PCP (Pneumocystis jirovecii pneumonia) — treatment and prophylaxis
  • Also used for: Toxoplasma, Nocardia, Stenotrophomonas, Isospora
  • HIV-exposed infants: give PCP prophylaxis from 6 weeks regardless of HIV status (per WHO)
  • High risk of hyperkalaemia with concurrent RAAS inhibitors

Contraindications

  • Sulfonamide allergy
  • Severe folate deficiency
  • Premature neonates (sulfonamides displace bilirubin — kernicterus)
  • First trimester pregnancy
  • Severe renal/hepatic impairment

Side effects

  • Rash (risk of SJS/TEN — stop immediately if blistering rash)
  • Bone marrow suppression
  • Hyperkalaemia (as per trimethoprim)
  • Hypersensitivity
  • GI upset

Interactions

  • Warfarin — significantly increased INR
  • Methotrexate — increased toxicity (folate antagonism)
  • ACE inhibitors/ARBs — hyperkalaemia
  • Phenytoin — toxicity

Monitoring

  • FBC (bone marrow suppression)
  • U&E (hyperkalaemia)
  • LFTs
  • Rash monitoring
  • Renal function

Reference: BNFc; BHIVA HIV Guidelines; WHO PMTCT Guidelines; BNF for Children. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

Calculators
Pathways