A virus called HIV causes AIDS, an illness that interferes with the body's ability to fight infection and some types of cancer. Transmission of HIV is caused by contact with blood or other body fluids that contain the virus. Treatments containing multiple drugs have improved outcomes for HIV-infected patients. These treatment combinations are known as highly active antiretroviral therapy (HAART). Efavirenz is 1 of the drugs used in HAART and is very effective at fighting HIV infection. However, about one half of persons who take efavirenz develop neuropsychiatric side effects, including trouble sleeping, nightmares, dizziness, depression, suicidal thoughts, hallucinations, and trouble thinking clearly. Although these symptoms usually go away after the first few weeks, some patients have to stop taking the drug because of them.