A pyrimidine inhibitor of dihydrofolate reductase, it is an antibacterial related to PYRIMETHAMINE. The interference with folic acid metabolism may cause a depression of hematopoiesis. It is potentiated by SULFONAMIDES and the TRIMETHOPRIM-SULFAMETHOXAZOLE COMBINATION is the form most often used. It is sometimes used alone as an antimalarial. TRIMETHOPRIM RESISTANCE has been reported.