In phase 4 (days 33 to 38), the steady-state metabolism and pharmacokinetics of efavirenz will be determined along the activities of CYP2B6, CYP2C8 and OATP1B1. The procedures described in phase 2 will be repeated. On day 38, an exit exam will be performed consisting of a repeat of a repeat of the screening laboratory tests including blood and urine tests. The study diary will be collected. Medication bottles with left over pills (if any) will be returned and pills remaining will be counted for compliance purposes. The total duration of the study will be 38 days.