Sulfamethoxazole/trimethoprim, also known as cotrimoxazole, Bactrim, Septra, or SMX-TMP, belongs to the class of medicines called antibacterials. It also belongs to the class of medicines called antiprotozoals. Antibacterials kill bacteria or stop bacteria from multiplying. Antiprotozoals help the body overcome infections caused by protozoa, which are microscopic organisms that can cause disease. When bacterial and protozoal infections occur in patients with weakened immune systems, including people with HIV, they tend to be more serious and are called opportunistic infections (OIs).