The study will randomize, in a 1:1 ratio, 260 recently detoxified opioid addicts beginning ART to a 48-week course of biweekly drug counseling with either oral naltrexone 50 mg/day + placebo naltrexone implant, or placebo tablet + naltrexone implant. Approximate balanced randomization will be achieved by using an urn randomization design within each set of sites based in two baseline characteristics: (1) viral load >100,000 copies/<100,000 copies, and (2) CDF count >50/>50 copies. The primary outcome will be the response to ART as measured by reduction in viral load at 24 and 48 weeks.