About Pyrimethamine Folic acid antagonist, Pyrimidine derivative, Antiprotozoal, Antimalarial. Mechanism of Action of Pyrimethamine Pyrimethamine binds to dihydrofolate reductase enzyme and inhibits the reduction of Dihydrofolic acid (DHFA) to Tetra hydrofolic acid (THFA) .This will interfere the synthesis of folic acid in malarial parasite.