All participants will be maintained on methadone throughout the study. Participants will also receive either the study medication (dextromethorphan, gabapentin, or oxycodone) or placebo four times per day beginning on Day 5. Study visits will take place once a week. At each visit, medications for the previous week will be accounted for and medications for the following week will be dispensed. Questionnaires and self-reports will be completed to assess depression levels and drug use. Urine tests will be used to screen for the presence of drugs and alcohol. During the study, participants will take part in four pain testing sessions to measure pain threshold and tolerance; two sessions will take place during Week 1 and another two sessions will occur during Week 5. The pain testing sessions will include a cold pressor (CP) test and an electrical stimulation (ES) procedure. The CP test will involve placing an arm in ice water for a short period of time; the ES procedure will involve stimulating muscle nerve endings via electrodes placed on the skin. Immediately after the CP and ES sessions, blood will be drawn for laboratory testing. Participants will again complete questionnaires and self-reports, and urine samples will be collected. Gift cards will be offered as an incentive for attending study visits and having negative drug tests.