pyrimethamine + sulfadiazine + leucovorin

This is a fixed combination therapy consisting of three drugs—pyrimethamine, sulfadiazine, and leucovorin—used primarily for the treatment of toxoplasmosis, including congenital, ocular, and cerebral forms. Pyrimethamine is a folic acid antagonist that inhibits the enzyme dihydrofolate reductase in *Toxoplasma gondii*, thereby blocking DNA synthesis. Sulfadiazine is a sulfonamide antibiotic that inhibits dihydropteroate synthase in the same pathway, providing synergistic antiparasitic activity with pyrimethamine. Leucovorin (folinic acid) does not have intrinsic antiparasitic activity but is co-administered to reduce the risk of bone marrow suppression and other hematologic toxicities caused by pyrimethamine’s inhibition of folic acid metabolism[1][2][4][7]. This regimen is considered first-line therapy for toxoplasmosis in both immunocompetent and immunocompromised patients (including those with HIV/AIDS), as well as for congenital toxoplasmosis[1][4][7]. The combination may also be used off-label or as an alternative regimen for prophylaxis or treatment of *Pneumocystis jirovecii* pneumonia when standard therapies are contraindicated[5][7].