During the 4-week induction period, participants were required to ingest scheduled doses of oral naltrexone to gain access to the therapeutic workplace. Induction onto oral naltrexone was determined by clinical judgment; participants were generally inducted using a 3-day dose run-up schedule (e.g., 12.5 mg, 25 mg, and 50 mg) in order to reach the full maintenance dose of 100 mg (Monday and Wednesday) and 150 mg (Friday) for the four-week period. Only participants who completed the induction period were considered eligible for randomization; these participants were invited to attend the therapeutic workplace for 26 weeks, and were offered oral naltrexone at no cost for the duration of the study. Participants were also offered access to outpatient drug abuse counseling that was provided independent of the workplace throughout their participation in the study.