Currently, the abuse of prescription opioid medications is a pervasive problem in the U.S. In addition, co-abuse of opioids and alcohol represents a significant problem from the perspective of increased toxicity and decreased success in treatment. Surprisingly few studies have examined the effects of combined administration of opioids and alcohol in humans, and no clinical studies have examined the reinforcing effects of this combination. The current 8-9-week inpatient study will systematically evaluate gabapentin because it shows promise for treating both opioid and alcohol use disorders (OUD and AUD). The guiding principle is that a medication's effects on positive subjective responses and reinforcing effects are the best laboratory procedures to date in predicting its clinical efficacy. We will examine the ability of gabapentin to alter opioid-, alcohol-,and combined opioid/alcohol-mediated responses. Participants will meet DSM-5 criteria for moderate-severe OUD and be physically dependent on opioids. In addition, participants will meet DSM-5 criteria for moderate-severe AUD, but they will not be physically dependent on alcohol. All of the participants will be maintained on oral morphine throughout the study and different doses of gabapentin will be evaluated.