Co-trimoxazole is used to treat certain bacterial infections, such as pneumonia (a lung infection), bronchitis (infection of the tubes leading to the lungs), and infections of the urinary tract, ears, and intestines. It also is used to treat ‘travelers’ diarrhea. Co-trimoxazole is a combination of trimethoprim and sulfamethoxazole and is in a class of medications called sulfonamides. It works by stopping the growth of bacteria. Antibiotics will not kill viruses that can cause colds, flu, or other viral infections.