Bactrim contains a combination of sulfamethoxazole and trimethoprim. Sulfamethoxazole and trimethoprim are are both antibiotics that treat different types of infection caused by bacteria. Bactrim is used to treat ear infections, urinary tract infections, bronchitis, traveler's diarrhea, shigellosis, and Pneumocystis jiroveci pneumonia. Bactrim may also be used for purposes not listed in this medication guide.