1.Pyrimethamine is an antimalarial drug. It prevents the growth and reproduction of organisms that cause malaria. 2.Pyrimethamine is used to treat and prevent malaria. 3.Pyrimethamine is also used together with a sulfa-based drug to treat an infection called toxoplasmosis. 4.Pyrimethamine may also be used for purposes other than those listed in this medication guide. finished medicine drug pharmaceutical