Antiretroviral Drug Interaction Study in Volunteers With HIV

Background: \- People who are infected with the human immunodeficiency virus (HIV) are at risk of getting certain diseases. Two of these diseases are a type of pneumonia known as PCP and a brain infection called toxoplasmosis. Most people with HIV take antiretroviral (ARV) drugs to treat HIV and lower the risk of infections. However, some ARV drugs may make other drugs used to treat PCP and toxoplasmosis less effective. Researchers want to test specific ARV drugs to see if they affect atovaquone, a drug used to treat PCP and toxoplasmosis. Objectives: \- To see if ARV drugs atazanavir-ritonavir or efavirenz lower the blood levels of atovaquone. Eligibility: * Individuals between 18 and 70 years of age who have HIV. * Participants must be taking efavirenz or atazanavir-ritonavir, or not taking any ARV drugs. Design: * Participants will be screened with a physical exam and medical history. They will also have blood and urine tests. * This study has a screening visit and five study visits. Two of the study visits will last about 12 hours; the other three visits will last about 1 hour each. * Participants will receive either a low dose or high dose of atovaquone to take for 14 days. They will record doses and any symptoms on a diary card at home. * After 14 days, participants will have a 12-hour visit to provide blood samples. There will be a wash-out period with no doses for up to 6 weeks. * After the wash-out period, participants will switch dose levels to either the high or low dose. * After 14 days, participants will have a 12-hour visit to provide blood samples.