Pyrimethamine is a medication primarily used to treat and prevent toxoplasmosis, a parasitic infection caused by the Toxoplasma gondii parasite. It works by inhibiting the growth of the parasite, making it effective in managing infections, especially in individuals with weakened immune systems, such as those with HIV/AIDS. In addition to treating toxoplasmosis, pyrimethamine is sometimes used in combination with other medications to treat malaria. It is important to follow a healthcare provider's instructions when using this medication, as it can have side effects and may require monitoring of blood cell counts during treatment.