Pyrimethamine is a medication

Medication

A pharmaceutical drug, also referred to as medicine, medication or medicament, can be loosely defined as any chemical substance intended for use in the medical diagnosis, cure, treatment, or prevention of disease.- Classification :...

 used for protozoal infections. It is commonly used as an antimalarial drug

Antimalarial drug

Antimalarial medications, also known as antimalarials, are designed to prevent or cure malaria. Such drugs may be used for some or all of the following:* Treatment of malaria in individuals with suspected or confirmed infection...

 (for both treatment and prevention of malaria

Malaria

Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

), and is also used (combined with sulfadiazine

Sulfadiazine

Sulfadiazine is a sulfonamide antibiotic.-Uses:It eliminates bacteria that cause infections by stopping the production of folic acid inside the bacterial cell, and is commonly used to treat urinary tract infections ....

) in the treatment of Toxoplasma gondii

Toxoplasma gondii

Toxoplasma gondii is a species of parasitic protozoa in the genus Toxoplasma. The definitive host of T. gondii is the cat, but the parasite can be carried by many warm-blooded animals . Toxoplasmosis, the disease of which T...

infections in immunocompromised patients, such as HIV

HIV

Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...

-positive individuals.
Pyrimethamine interferes with tetrahydrofolic acid

Tetrahydrofolic acid

Tetrahydrofolic acid, or tetrahydrofolate, is a folic acid derivative.-Metabolism:-Human synthesis:It is produced from dihydrofolic acid by dihydrofolate reductase...

 synthesis from folic acid

Folic acid

Folic acid and folate , as well as pteroyl-L-glutamic acid, pteroyl-L-glutamate, and pteroylmonoglutamic acid are forms of the water-soluble vitamin B9...

 by inhibiting the enzyme dihydrofolate reductase

Dihydrofolate reductase

- Function :Dihydrofolate reductase converts dihydrofolate into tetrahydrofolate, a methyl group shuttle required for the de novo synthesis of purines, thymidylic acid, and certain amino acids...

 (DHFR). Tetrahydrofolic acid is needed for DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 and RNA

RNA

Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

 synthesis in many species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

, including protozoa.
Resistance to pyrimethamine is widespread.