Current treatments for opioid addiction would benefit by the addition of a non-opioid based treatment medication. Recent behavioral studies have shown that the neurokinin-1 (NK1) receptor is involved in opioid reward and withdrawal. This study proposes to study a potential non-opioid treatment, the clinically available, FDA approved, NK1 antagonist aprepitant, in opioid addicted patients. Based on the unique behavioral and pharmacological characteristics of opioid addiction, and what is known of the currently employed treatments, the investigators propose that the therapeutic mechanism of any potential opioid addiction treatment medication must include the ability to reduce opioid withdrawal. This is of particular importance during treatment initiation (eg. detoxification). In addition, for long-term treatment and relapse prevention, it is important to manage drug craving and inhibit the rewarding effects of opioids if patients do experience a slip. Therefore, the investigators propose to study aprepitant using human models of opioid withdrawal, craving and acute opioid reward and reinforcement. The investigators will also include a neuro-economics choice procedure paradigm.