A synthetic combination of two antibacterial agents, trimethoprim and sulfamethoxazole. This synergistic combination, also known as co-trimoxazole, inhibits two sequential steps in the bacterial metabolism of folic acid. Trimethoprim is a pyrimidine inhibitor of dihydrofolate reductase; sulfamethoxazole is a sulfamide inhibitor of bacterial dihydrofolate synthetase.