Participants in this open label study will receive atomoxetine for 12 weeks, and will take one dose each morning for the duration of the study. Doses will be increased gradually to minimize side effects and enhance treatment compliance. In addition, all participants will receive individualized relapse prevention therapy once weekly. Participants will be required to report to the study site three times a week to receive medication, complete questionnaires regarding ADHD symptoms and substance use behavior, and provide a urine sample while being supervised by study staff. Also, vital signs will be monitored and medication side effects will be assessed at each visit. Participants will report to the study site 6 months after starting in the study for a follow-up visit, at which time ADHD symptoms, substance use behavior, and social functioning will be assessed.