Co-trimoxazole (Trimethoprim + Sulfamethoxazole)

How it works

The two components block sequential steps in bacterial folate synthesis, producing a synergistic effect.

Prescribing in practice

• It carries sulfonamide risks — serious skin reactions (including Stevens-Johnson syndrome) — and can cause hyperkalaemia and bone-marrow suppression.
• Avoid or use great caution with methotrexate (additive folate antagonism and marrow toxicity) and with other potassium-raising or marrow-suppressing drugs.
• Reduce the dose in renal impairment and ensure good hydration; it is usually reserved for specific indications.

Monitoring

For longer courses monitor FBC, renal function and potassium, and review for rash.

Counselling the patient

• Report any rash promptly, or a sore throat or fever.
• Drink plenty of fluids.
• Tell your clinician about other medicines, especially methotrexate.

Evidence & guidelines

First-line for Pneumocystis pneumonia treatment and prophylaxis, and used selectively for other infections per local guidance.