Efavirenz is a non-nucleoside reverse transcriptase inhibitor (NNRTI) which shows good inhibitory activity against HIV-1. Reduced susceptibility to efavirenz has been reported with HIV-1 variants containing single and multiple mutations to the reverse transcriptase enzyme. In vitro and in vivo data suggest that the resistance profile of efavirenz overlaps with that of the NNRTIs nevirapine and delavirdine. Clinically significant drug interactions have been reported with efavirenz and indinavir and saquinavir. An increase in dosage of indinavir from 800 to 1000 mg 3 times daily is recommended during coadministration with efavirenz. Use of efavirenz in combination with saquinavir as the sole protease inhibitor is not recommended. Once-daily efavirenz in combination with zidovudine plus lamivudine or indinavir or nelfinavir increased CD4+ cell counts and reduced HIV RNA plasma levels to below quantifiable levels (< 400 copies/ml) in HIV-infected patients. A sustained reduction in viral load was maintained for at least 72 weeks in 1 study. Nervous system symptoms (including headache, dizziness, insomnia and fatigue) and dermatological effects (including maculopapular rash) appear to be the most common adverse events reported with efavirenz-containing antiretroviral regimens.