Individuals addicted to cocaine often experience feelings of restlessness, irritability, anxiety, and paranoia. Reducing cocaine use can lead to depression. In an attempt to alleviate the depressive symptoms, individuals may resume drug use. Fluoxetine, a medication that is currently used to treat depression and panic attacks, may assist at reducing cocaine use in individuals addicted to cocaine. By increasing serotonin, a chemical in the brain that helps maintain mental balance, fluoxetine may lessen depressive symptoms and decrease the reinforcing effects of cocaine. Voucher incentive programs, in which items or services are offered as a reward for remaining drug-free, are also a useful component of substance abuse treatment. Preliminary research has shown that fluoxetine, when combined with an incentive program, is an effective treatment for reducing cocaine use. Further research is needed to confirm the benefits of fluoxetine for cocaine and opioid addicts. The purpose of this study is to evaluate the effectiveness of fluoxetine, in combination with a voucher incentive program, at reducing cocaine use in methadone-maintained cocaine and opioid addicts.