Participants in this double blind study will be randomly assigned to receive either disulfiram at one of three dosage levels (62.5, 125, 250 mg/day) or placebo for 14 weeks. During the first 2 weeks, doses of methadone plus either disulfiram or placebo will be administered until the assigned maintenance dose is achieved. This dosage level will be maintained for the duration of the study. Participants will be required to report to the study site Monday through Saturday of each week for assessments and to receive medication. They will receive a take-home dose of medication for Sunday. In addition, participants will receive a weekly session of cognitive-behavioral therapy. Urine samples will be collected three times a week to assess drug use. At the end of the study, participants will undergo a 4- to 6-week detoxification period in which they will gradually discontinue the use of methadone and disulfiram. All participants have the option to request transfer to a local methadone maintenance program instead of undergoing detoxification.