Naltrexone is a medication that has been shown to help prevent relapse to opioid addiction and it has been reported to be clinically effective in parolee populations although it is rarely used. Recently a depot formulation with one-month duration has received FDA approval for the treatment of alcoholism and opiate dependence. This means that rather than having to take medication daily, individuals can receive one injection that lasts for approximately 30 days. The purpose of this study is to determine whether this monthly injection of naltrexone is practical and useful in the prevention of relapse to opioids and re-incarceration when administered to inmates prior to release from prison. The investigators will also monitor HIV risk behaviors to determine whether the intervention reduces risky behaviors associated with intravenous drug abuse and the spread of viruses such as HIV and hepatitis C. Volunteers will be randomized to receive an injection of depot naltrexone prior to release from prison or to contact study personnel in the community following release. Participants assigned to receive naltrexone in prison will receive 1 injection in prison, and 5 additional monthly injections for 5 months upon release. Participants assigned to contact study personnel upon release will receive all 6 injections in the community at RIH after their release from the ACI. Patients in both groups will be given identical follow up monthly for six months including measures of opiate use by self-report, and urine tests. An additional scheduled urine test will take place each month between monthly visits. There will also be a 12-month follow-up period for participants in both groups, which will consist of 2 visits, spaced 6 months apart, meaning that participants will be enrolled in the study for a total of about 18 months. All participants will be asked to complete brief questionnaires at follow-up visits to assess things such as services received, drug use, and depression.