Co-Trimoxazole

How it works

The two components block sequential steps in bacterial folate synthesis, trimethoprim inhibiting dihydrofolate reductase and sulfamethoxazole inhibiting dihydropteroate synthase, giving a synergistic bactericidal effect.

Prescribing in practice

• Avoid in infants under the threshold age stated in current prescribing references because of the risk of kernicterus, and in glucose-6-phosphate dehydrogenase deficiency where it can precipitate haemolysis.
• Rare but serious skin reactions including Stevens-Johnson syndrome and toxic epidermal necrolysis can occur, so stop at the first sign of rash or mucosal involvement.
• Dose by body weight using a children's formulary and reduce in renal impairment; ensure adequate fluid intake to limit crystalluria.

Monitoring

On prolonged courses monitor full blood count, renal function, electrolytes (particularly potassium) and watch for rash, as bone marrow suppression and hyperkalaemia can occur.

Counselling the patient

• Seek urgent help if a rash, blistering, mouth ulcers, unexplained bruising, sore throat or fever develops.
• Complete the full course and maintain good fluid intake during treatment.
• Tell the team about any history of sulphonamide allergy or G6PD deficiency.

Evidence & guidelines

Co-trimoxazole is the established first-line agent for Pneumocystis jirovecii pneumonia prophylaxis in immunocompromised children, supported by long-standing UK and international guidance.