Efavirenz is an antiretroviral medicine that slows the progress of human immunodeficiency virus (HIV) infection; it is not a cure. HIV destroys cells in the body, called CD4 T cells. These cells are a type of white blood cell and are important because they are involved in protecting your body from infection. If left untreated, the HIV infection weakens your immune system so that your body cannot defend itself against bacteria, viruses and other germs. Efavirenz slows down the progress of HIV infection by reducing the amount of virus in your body. It does this by stopping the virus from copying (replicating) itself.