This 33-week study will enroll individuals addicted to both cocaine and opioids who are concurrently enrolled in a methadone treatment program. Participants will be randomly assigned to receive either 60 mg of fluoxetine or placebo on a daily basis for 12 weeks from Weeks 8 through 19. Study visits will occur once a week. Questionnaires and self-reports will be completed to assess medication side effects and cocaine use. Cocaine levels will be monitored three times a week with urine tests. During the 12 weeks of treatment, participants will also be randomly assigned to either take part in a voucher incentives program or not take part in the program. Participants in the voucher program will receive a voucher that may be exchanged for various items and services for each urine test with a negative cocaine result.