This is due to allergy to Bactrim. When you have allergies, your immune system makes antibodies that identify a particular allergen as harmful, even though it isn t. What you got is serum sickness type of allergic reaction. Advice 1. Stop Bactrim permanently and do not take it in future 2. Paracetamol 500 mg orally if fever occurs 3. Hydroxyzine 10 mg (Atarax 10 mg) every 8 hours per oral for 7 days 4. Ranitidine 150 mg twice daily per oral for 10 days 5. Do not scratch too much as it will caused skin damage and infection 6. Do gargle with slight warm salt water four times daily 7. Calamine lotion – three times daily locally – will provide soothing effect 8. Drink plenty of water at least 4 –5 liters daily 9. Wear loose-fitting smooth cotton fabrics Thanks