Pentamidine and atovaquone are antiprotozoal agents that are primarily used to treat Pneumocystis pneumonia. Pentamidine is discussed in Chapters 31-01 and 35-01. Atovaquone inhibits mitochondrial electron transport and probably also folate metabolism. The solid dosage form is poorly absorbed and should be given with food to maximize bioavailability. The suspension is significantly better absorbed and preferred especially in high-risk patients (those with diarrhea, malabsorption). It has moderate activity against P jirovecii. In comparative trials with trimethoprim-sulfamethoxazole and pentamidine in the therapy of Pneumocystis pneumonia in HIV/AIDS, atovaquone, 750 mg orally three times daily for 3 weeks, is less effective than both agents but better tolerated. It has also been used as prophylaxis in HIV/AIDS patients at a dosage of 1500 mg daily. Major adverse effects include rash, nausea, vomiting, diarrhea, fever, and abnormal liver biochemical tests. The use of atovaquone is limited to patients with mild to moderate Pneumocystis infections who have not responded to or cannot tolerate other therapies.