The opioid system plays a role in diverse biological functions, including reward, analgesia, and stress responsivity (Kreek and Koob, 1998; Vaccarino and Kastin, 2000). Therapeutically, opioids are commonly prescribed for their effective analgesic properties. However, the response to treatments varies widely between individuals leaving many people taking the wrong dose, experiencing unbearable side effects, or receiving inadequate therapy. Additionally, chronic use is marred by habituation, tolerance, and the development of dependence, which occur in varying degrees depending on the individual. The ability to better predict clinical outcomes based on individual differences to opioid therapeutics would greatly reduce the trial and error of finding suitable drugs and doses, and could reduce the number of patients developing drug dependence.
