An antibiotic (Not for Human Consumption) used to prevent and treat bacterial infections. It consists of one part trimethoprim to five parts sulfamethoxazole. It is used for urinary tract infections, MRSA skin infections, travelers' diarrhea, respiratory tract infections, and cholera, among others. It may be used both to treat and prevent pneumocystis pneumonia in people with HIV/AIDS.

Common side effects include nausea, vomiting, rash, and diarrhea. Severe allergic reactions and Clostridium difficile diarrhea may occasionally occur. Its use near the end of pregnancy is not recommended. It appears to be okay during breastfeeding as long as the baby is healthy. TMP/SMX generally results in bacterial death. It works by blocking the making of folate by the bacteria.