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.

Inclusion Criteria:

1. Incarcerated adults with known release date.
2. Meet DSM-IV criteria for current (and/or prior to incarceration) opioid dependence.
3. Not interested in agonist (methadone, buprenorphine) treatment.
4. Currently opioid free by history ('detoxed'), with negative urine for all opioids and no sign of opiate withdrawal after IV (or IM if no available venous access) injection of 0.8 mg naloxone.
5. Good health by medical history, physical examination and laboratory tests.
6. Age older than 18.
7. Understands and signs a consent form.
8. Able to speak and understand English.
9. Females: not pregnant (urine hCG negative at baseline and prior to each injection), not planning conception; and planning appropriate contraception if sexually active.

Exclusion Criteria:

1. Current drug or alcohol dependence requiring long term residential treatment that would interfere with outpatient study participation. Dependence on substances that commonly co-occur with opioid dependence (e.g. cocaine, cannabis, alcohol) that do not rise to this level of severity will not be exclusionary in order to achieve a maximally representative sample.
2. Liver failure and/or liver function test levels greater than three times normal.
3. Pregnancy, lactation, or failure to use adequate contraceptive methods;
4. Active medical illness that might make participation hazardous, e.g. untreated hypertension, hepatitis with AST or ALT > 3 times upper limit of normal, unstable diabetes or heart disease. Adequately treated medical conditions are acceptable.
5. Untreated psychiatric disorder that might make participation hazardous, e.g. untreated psychosis, bipolar disorder with mania, significant suicide risk. Adequately treated psychiatric disorders and appropriate psychotropic medications would be allowed.
6. History of allergic reaction to naltrexone;
7. Current chronic pain diagnosis for which opioids are required for pain relief.
8. Obesity (BMI of 40 or greater) to reduce the likelihood of injection site reaction.
9. Known intolerance and/or hypersensitivity to naltrexone, carboxymethylcellulose, or polylactide-co-polymers (PLG) or any other components of the diluents.