Pyrimethamine is a synthetic antiparasitic API (active pharmaceutical ingredient) belonging to the diaminopyrimidine class. It functions as a competitive inhibitor of dihydrofolate reductase, disrupting folate metabolism crucial for DNA synthesis in susceptible parasites. Primarily used in combination with sulfonamides (e.g., sulfadoxine), it is a cornerstone in the treatment and prophylaxis of malaria caused by�Plasmodium falciparum. Its other key indication is the treatment of toxoplasmosis, particularly in immunocompromised patients. Due to the risk of folate deficiency and hematological toxicity (e.g., megaloblastic anemia), administration often requires concomitant folinic acid (leucovorin) rescue. It is typically administered orally.