This study will last 11 weeks. Participants will stay in a residential research unit for the duration of the study. Participants will be randomly assigned to receive different doses of daily, sublingual buprenorphine/naloxone. After a minimum of 2 weeks on each dose, participants will undergo challenge sessions on each weekday for 1 week (Monday through Friday). Challenge sessions will consist of increasing doses of intramuscular hydromorphone. During the challenge session week, buprenorphine/naloxone will be given only on Monday; a placebo will be given the rest of the week. Challenge sessions will examine the blockade effects of buprenorphine/naloxone at 2, 26, 50, 74, and 98 hours after the last active dose of buprenorphine/naloxone.