To address the question of the comparison of two courses of Vivitrol with differing lengths in 130 HIV negative, consenting, opioid addicted patients who have completed inpatient treatment. Participants will be randomized under double blind conditions to a 24 or 48-week course of pharmacotherapy, along with bi-weekly drug counselling, over 48 weeks, with follow-ups at weeks 60 and 72. The 24-week cohort will receive Vivitrol placebo injections in weeks 24 to 48.