Sulfadiazine

How it works

It is a structural analogue of para-aminobenzoic acid and competitively inhibits dihydropteroate synthase, blocking bacterial folate synthesis and exerting a bacteriostatic effect.

Prescribing in practice

• Stop immediately and seek urgent advice if a rash develops, as sulfonamides can cause severe cutaneous reactions including Stevens-Johnson syndrome and toxic epidermal necrolysis.
• Maintain a good fluid intake to reduce the risk of crystalluria; it is contraindicated in known sulfonamide hypersensitivity.
• Use with caution in G6PD deficiency owing to the risk of haemolysis, and avoid near term in pregnancy because of the risk of neonatal kernicterus.

Monitoring

Monitor the full blood count during prolonged therapy and review renal function and hydration given the risk of crystalluria.

Counselling the patient

• Report any rash, mouth ulcers, sore throat, fever or bruising promptly.
• Drink plenty of fluids throughout the course.
• Tell your team about any previous reaction to a 'sulfa' medicine.

Evidence & guidelines

Its use in toxoplasmosis with pyrimethamine and in rheumatic fever prophylaxis is long-established in UK and international guidance and prescribing references.