2. Subjects who test positive on the urine drug screen for any illicit drugs other than cocaine and marijuana during screening will be allowed a single retest. Those individuals who test positive for amphetamine during screening, given that they provide a copy of a prescription, will only be included if they can safely discontinue amphetamine use for the duration of the study. Subjects will need to provide a urine free of all illicit drugs other than cocaine and marijuana at study onset to be randomized. Subjects who test positive for any drugs other than marijuana prior to a study session will be allowed a single retest and a chance to reschedule their session. If the subject tests positive for any drug other than marijuana at the retest, their participation in the study will be terminated.