Sulfamethoxazole and trimethoprim will not treat a viral infection (flu or a common cold). You may need blood and urine tests, and this medicine may be stopped based on the results. Store at room temperature away from moisture, heat, and light. Sulfamethoxazole with trimethoprim is a fixed antibiotic combination that is widely used for mild-to-moderate bacterial infections and as prophylaxis against opportunistic infections. Like other sulfonamide-containing medications, this combination has been linked to rare instances of clinically apparent acute liver injury.