A number of medications closely related to the sulfonamides (eg, dapsone) have been used effectively in the long-term treatment of leprosy. The clinical manifestations of both lepromatous and tuberculoid leprosy can often be suppressed by treatment extending over several years. At least 5–30% of Mycobacterium leprae organisms are resistant to dapsone, so initial three-drug treatment with rifampin, dapsone, and clofazimine is advocated. Dapsone, 100 mg daily, is effective therapy for mild to moderate Pneumocystis pneumonia in AIDS when combined with trimethoprim, 15 mg/kg/day in three divided doses. At a dose of 50–100 mg daily or 100 mg two or three times a week, it is effective prophylaxis for P jirovecii infection and, when combined with pyrimethamine, 50 mg per week, also prevents Toxoplasma encephalitis in HIV-infected patients.