In phase 3 (days 16 to 32), subjects will take efavirenz (600 mg/day) every evening for 17 days (Note: Phase 3 starts when the subject takes the first dose that evening on Day 16). Prior to taking the evening dose, they will be requested to return to the Indiana Clinical Research Center on day 19, day 22, day 25, day 28 and day 31 for a brief outpatient visit to have vital signs checked, draw blood sample (~10 ml, approximately 2 teaspoonful), and to fill out the brief central nervous system (CNS) symptoms questionnaire. They will be reminded not to take the evening dose of efavirenz on these days until after their blood draw. Since efavirenz has a long half-life (~76 hours after single dose and ~50 at steady state), a total of 17 days of efavirenz treatment is required to achieve steady-state plasma concentrations of efavirenz and steady-state autoinduction of metabolism. Subjects will be instructed to take efavirenz at bed-time to minimize efavirenz-induced central nervous side effects. Minor rescheduling (±3 days) of the subject's fixed inpatient/outpatient visits will be allowed including adjusting the amount of study medication (up to 3 doses less or up to 3 doses more) to be given prior to the subject's 4th inpatient visit.