Ormetoprim is a diaminopyrimidine-based inhibitor of dihydrofolate reductase, the enzyme responsible for NADPH-dependent reduction of 7,8-dihydrofolate to 5,6,7,8-tetrahydrofolate. (1) This inhibition results in the interference in folic acid production. In bacteria, Ormetoprim, weakly antibacterial by itself, can act as a potentiator by cotreatment with sulfadimethoxine, which in turn prevents the formation of folinic acid. (2)