This study will last 10 weeks. Participants will stay in a residential research unit, and will be maintained on oral hydromorphone (10 mg). During twice-weekly experimental sessions, participants will be randomly assigned to receive either sublingual tablets, intramuscular injections, or a placebo. The 15 conditions studied will include: sublingual or intramuscular buprenorphine/naloxone (1/0.25 mg, 2/0.5 mg, 4/1 mg, 8/2 mg, and 16/4 mg), 0.25 mg of intramuscular naloxone (antagonist control), 10 mg of intramuscular hydromorphone (agonist control), sublingual and intramuscular buprenorphine (8 mg), and placebo.