Trimethoprim + sulfamethoxazole

Co-trimoxazole

Brand names: Septrin

Adult dose

Dose: Standard: 960mg BD. PJP treatment: 120mg/kg/day in 2–4 divided doses for 14–21 days. PJP prophylaxis: 480–960mg OD or thrice weekly

Route: PO/IV

Frequency: BD

Paediatric dose

Dose: 24 mg/kg

Route: PO

Frequency: BD

Max: 960mg/dose

Standard 24 mg/kg BD; PJP treatment 60 mg/kg BD

Dose adjustments

Renal

CrCl 15–30: half dose; avoid if <15 unless dialysed

Paediatric weight-based calculator

Patient weight (kg)

Standard 24 mg/kg BD; PJP treatment 60 mg/kg BD

Clinical pearls

Restricted use in UK (CSM advice): primarily PJP, toxoplasmosis, nocardiosis, where alternatives unsuitable
Maintain hydration

Contraindications

Severe renal/hepatic impairment
Pregnancy (esp. 1st trimester and term)
Folate deficiency
G6PD deficiency
Premature/jaundiced neonates
Acute porphyria

Side effects

Rash (incl. SJS/TEN)
Hyperkalaemia
Bone marrow suppression
Nausea
Hepatotoxicity
Crystalluria/AKI
Hyponatraemia

Interactions

Warfarin (raised INR)
Methotrexate (toxicity)
Phenytoin (raised levels)
ACEi/ARB/spironolactone (hyperkalaemia)
Sulfonylureas (hypoglycaemia)

Monitoring

U&E
FBC
LFTs
Folate if prolonged

Reference: BNFc; BNF; CSM advice; https://bnf.nice.org.uk/drugs/co-trimoxazole/. Verify against your local formulary and the latest BNF before prescribing.

Curated clinical cross-links plus same-class fallbacks.

Pathways