All participants in this study received Medisorb naltrexone at double-blinded dose strengths (ie, 190 mg or 380 mg); no participant received placebo. Participants who had received Medisorb naltrexone in Study ALK21-003 (NCT01218958) continued to receive the same dose strength in this extension study. Those who had received placebo for Medisorb naltrexone 190 mg in the base study were given Medisorb naltrexone 190 mg. Participants who had received placebo for Medisorb naltrexone 380 mg in the base study were given Medisorb naltrexone 380 mg.