- Subjects must be an experienced opioid user who meets the following criteria: 1) has used opioids for non-therapeutic purposes (ie, for psychoactive effects) on at least 10 occasions in the past five years; 2) has used opioids at least 2 times in the two years prior to screening; and 3) is not physically dependent on opioids, as assessed by medical history and naloxone challenge performed at screening