This study will involve three separate experiments. Participants in Experiment 1 will be randomly assigned to receive either dextromethorphan or placebo; in Experiment 2, participants will be randomly assigned to receive either gabapentin or placebo; and in Experiment 3, participants will be randomly assigned to receive either oxycodone or placebo. Each experiment will last 5 weeks. All 3 experiments will begin with a screening session. Potential participants will undergo a physical exam and an electrocardiogram. Blood and urine will be collected for laboratory tests and drug screening. Each individual's medical and drug history will be reviewed and psychological and opiate withdrawal symptoms will be assessed. Individuals who complete the screening and meet all study requirements will be permitted to continue in the study.