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Trimethoprim + sulfamethoxazole

Co-trimoxazole

Brand names: Septrin

Co-trimoxazole is a fixed combination of trimethoprim and sulfamethoxazole, used principally for the treatment and prevention of Pneumocystis jirovecii pneumonia and for selected other infections where it is the agent of choice.

Dosing — being independently re-sourced

ClinCalc Pro is rebuilding its dose data from primary open sources — the manufacturer SmPC (eMC), the WHO Model Formulary and other official references — under clinician review. This drug's structured dose is not yet published here. Confirm all doses against the product SmPC and your local formulary before prescribing.

Clinical monograph

How it works

The two components block sequential steps of bacterial folate synthesis — sulfamethoxazole inhibits dihydropteroate synthase and trimethoprim inhibits dihydrofolate reductase — giving a synergistic effect.

Prescribing in practice

  • It can cause life-threatening reactions including severe cutaneous reactions such as Stevens-Johnson syndrome, blood dyscrasias and hyperkalaemia, so use is restricted to specific indications.
  • It raises the risk of hyperkalaemia, particularly with renal impairment or alongside ACE inhibitors, angiotensin receptor blockers or potassium-sparing diuretics.
  • It potentiates warfarin and methotrexate and should be avoided with methotrexate because of additive antifolate marrow toxicity.

Monitoring

Monitor full blood count, renal function and serum potassium during prolonged or high-dose therapy.

Counselling the patient

  • Report any rash, mouth ulcers, sore throat, fever or unusual bruising immediately.
  • Maintain a good fluid intake during treatment.

Evidence & guidelines

MHRA guidance restricts co-trimoxazole to indications where benefit outweighs the risk of serious skin, blood and electrolyte reactions.

Reference: CSM advice; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

Pathways