Participants in this 21-day, double-blind study will be randomly assigned to receive either bupropion or placebo. Treatment with bupropion will occur for one week before stopping marijuana use (quit day). Bupropion will be administered at 150 mg per day for 3 days, followed by 300 mg per day for the remainder of the study. Quit day will occur on Day 8, at which time participants will be required to remain abstinent from marijuana for the duration of the study. Participants will provide daily urine specimens. Changes in vigilance, memory, reaction time, and psychomotor performance as well as withdrawal symptoms will be evaluated.