Participants in this observational study will undergo multiple test sessions in which they will select between acquiring drug or money. Because participants may choose money on all 12 trials of test sessions as a way to avoid receiving an injection, a placebo injection will be given when money is chosen. Prior to each test session, participants will receive a sample of the drug dose. During each test session, participants will have 12 opportunities to select either drug (administered as HYD) or money. Prior to each individual choice trial, participants will receive supplemental HYD. Participants will use a computer to earn choices. Respiration rate, oxygen saturation, heart rate, and blood pressure will be monitored throughout choice trials. Pupil diameter will be measured with a digital camera. Participants will complete self-report questionnaires at different times during the study. Participants will be maintained on buprenorphine throughout the study, with a fixed 3-week detoxification after study completion.