This study focuses on whether the addition of mobile medical extended-release naltrexone treatment (XR-NTX) will increase adherence and thus efficacy of the medication for pre-release prisoners. Project implementation occurs at 9 prisons under the jurisdiction of the Maryland Department of Public Safety and Correctional Services (MD DPSCS). Following initial screening, informed consent, and medical examination, consenting individuals at each facility are block randomized within gender, to either XR-NTX-OTx: One injection of XR-NTX in prison, followed by six monthly injections in the community post-release at an opioid treatment program (OTP); or XR-NTX+MMTx: One injection of XR-NTX in prison, followed by six monthly injections in the community post-release via mobile services at the participant’s place of residence. All participants are evaluated monthly for 7 months with follow up at 1-, 2-, 3-, 4-, 5-, 6-, 7- (safety visit) and 12-months after release from prison. We consider the follow-up schedule advantageous from both a treatment and research perspective. The proposed study has two specific aims: Aim 1: To compare the two study conditions in terms of: a) XR-NTX treatment adherence; b) opioid use; c) criminal activity; d) re-arrest; e) re-incarceration; and f) HIV risk-behaviors (i. needle use; ii. risky sexual behaviors). Aim 2: To determine if the number of months of post-release XR-NTX treatment is related to outcomes, and if so, is there a point at which XR-NTX v. Non-XR-NTX equilibrates. Such a finding could be potentially important because it would be informative about the needed length of XR-NTX treatment.