Bactrim Side Effects. Severity: Major. You should check with your doctor immediately if any of these side effects occur when taking sulfamethoxazole / trimethoprim:
- Abdominal or stomach pain
- black, tarry stools
- blistering, peeling, or loosening of the skin
- changes in skin color
- chest pain
- chills
- cough or hoarseness
- dark urine
- diarrhea