Sustiva (efavirenz) is a non-nucleoside reverse transcriptase inhibitor (NNRTI) , effective only for HIV type 1 (the most common type of HIV).
HIV is a retrovirus: it holds all its genetic information in the form of RNA. When it infects a host cell, the RNA gets copied into DNA by an enzyme called reverse transcriptase. In the form of DNA, the viral genetic code can integrate itself in its hostís own DNA. Then, as the hostís DNA is copied into RNA and then proteins are produced according to the code, the viral DNA gets copied along, and produces its own proteins. The viral proteins together with viral RNA then join to make up more viruses. HIV-1 inhibits the reverse transcriptase enzyme of HIV-1, leading to a reduction of HIV-1 levels in the body and an increase in the levels of CD4+ cells (cells of our immune system and one of the main targets of HIV Ė their levels normally decrease after an HIV infection).