Potential subjects were screened within 21 days prior to dosing of study drug (Medisorb naltrexone or placebo) on Day 0. Screening evaluations included a baseline hydromorphone challenge session in which increasing doses of hydromorphone (0 mg [placebo], 3 mg, 4.5 mg, and 6 mg) were administered at hourly intervals to produce a cumulative dose-response curve. Throughout the 4-hour challenge period, subject-rated measures (Visual Analog Scale [VAS] questions) and physiological measures (ie, pupil size) were recorded.