Pyrimethamine (CAS 58-14-0) is a synthetic heterocyclic compound with the molecular formula C₁₂H₁₃ClN₄ and molecular weight 248.71 g/mol. It functions as a potent inhibitor of dihydrofolate reductase (DHFR) and is supplied as a high-purity (99-100%) analytical reference standard for laboratory use in pharmaceutical analysis, quality control, and biochemical research.

Pyrimethamine is primarily used as an antiparasitic medication, particularly in the treatment and prevention of malaria. It is often combined with sulfadoxine to enhance its efficacy against the Plasmodium parasite, which causes malaria. This combination therapy is widely used in areas where malaria is endemic, especially for treating uncomplicated cases.

In addition to malaria, pyrimethamine is also employed in the treatment of toxoplasmosis, a parasitic infection caused by Toxoplasma gondii. It is particularly crucial for patients with weakened immune systems, such as those with HIV/AIDS, to prevent severe complications from the infection. The drug works by inhibiting the synthesis of folic acid in the parasite, which is essential for its survival and replication.

Pyrimethamine is sometimes used in veterinary medicine to treat parasitic infections in animals. Its ability to target and disrupt the life cycle of various parasites makes it a valuable tool in both human and animal health. However, its use must be carefully monitored, including periodic blood level checks, due to potential side effects and the risk of resistance development in parasites.