Next, participants will live on an inpatient research unit for at least 11 consecutive nights and possibly up to 15 consecutive nights. During this stay they will participate in a total of eight experimental sessions. Participants will take part in multiple trials in which they have the opportunity to choose drug, hydromorphone (HYD), or money. HYD is a drug that is currently used as a cough suppressant and to relieve pain. During the first two test sessions, participants will receive a sample of the drug doses that can be chosen. Before each of the final 6 test sessions begin, participants will be given a capsule containing either different doses of the drug yohimbine or a placebo (blank). Yohimbine is a drug that has been shown to produce a "stress"-like response in humans. Then participants will have 12 opportunities to choose either drug or money by using a computer to earn choices. Respiration rate, oxygen saturation, heart rate, and blood pressure will be monitored throughout choice trials. Self-report questionnaires will be completed at different times during the study.