Trimethoprim is a bactericidal antibiotic which acts by interfering with the action of bacterial dihydrofolate reductase, inhibiting synthesis of tetrahydrofolic acid. It is most commonly used in combination with sulfamethoxazole, which inhibits the formation of folic acid from para-aminobenzoic acid (PABA). The two drugs, given orally or intravenously in a fixed ratio, are manufactured under the trade-names Bactrim, Septra, Cotrim, or in generic formulation, and act synergistically by inhibiting successive steps in folate metabolism.

The medication is commonly used in urinary tract infections and in pneumocystis carinii pneumonia.

Further information
Nucleic Acid Inhibitors