In phase 2 (days 11 to 16), the metabolism and pharmacokinetics of a single 600 mg oral dose efavirenz will be determined along with the activities of CYP2B6, CYP2C8 and OATP1B1. Subjects receive a single 600 mg oral dose of efavirenz and blood samples will be obtained at 30 min and 1 hour. Immediately after the 1 hour blood draw, the three probe drugs (10 mg montelukast, 100 mg bupropion, and 5 mg rosuvastatin) will be administered simultaneously. Frequent plasma samples will be collected for 120 hours and urine for 48 hours after efavirenz dosing. Safety assessment will be made for the first 24 hours. Phase 2 is completed after the last blood draw on day 16 (120 hours). Note: The additional time points (compared to phase 1) are needed because of the long elimination half-life of efavirenz.