• Pain response (measured after each pain testing session on Days 1, 4, 36, and 39) [ Time Frame: 6 week ] [ Designated as safety issue: No ]
  

Drug: Gabapentin
Gabapentin initiated up to a daily dose of 2400mg PO x 5 weeks

Opioid medications are frequently used for the treatment of moderate to severe pain; however, individuals who use opioids have a high risk of becoming addicted. Opioid users who abruptly stop using opioid drugs may experience withdrawal symptoms, including drug craving, sweating, sleep disruption, nausea, irritability, and pain. Hyperalgesia, a severe and excessive sensitivity to pain, is a serious condition that may also occur when opioid use is reduced or discontinued. Opioid medications affect both the pain inhibitory and facilitatory systems, meaning that while they are effective at treating pain in many individuals, they also have the ability to intensify pain and cause hyperalgesia in some opioid users. Three medications, dextromethorphan, gabapentin, and oxycodone, may alleviate the symptoms of hyperalgesia and lessen an individual's sensitivity to pain. Further research is needed to confirm the benefits of these medications for opioid addicts. The purpose of this study is to compare the effectiveness of dextromethorphan, gabapentin, and oxycodone at reducing opioid-induced hyperalgesia in methadone-maintained opioid addicts.

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.

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.