Participants in this 12-week, double-blind, placebo-controlled trial will be randomly assigned to receive either lofexidine or placebo while currently receiving standard naltrexone outpatient treatment. Lofexidine will be initiated at twice daily doses of 0.4 mg and increased to 0.8 mg by the end of Week 1. The doses will be increased to 1.2 mg by the end of Week 2, and maintained at this level for Weeks 3 through 12. During Week 12, lofexidine discontinuation will be tapered over 4 days. Hour-long study visits will occur 3 times each week to assess vital signs, medication side effects, and withdrawal symptoms. Blood, alcohol, and urine tests will be performed as well as a psychiatric evaluation. Administration of naltrexone will also occur 3 times each week. Follow-up visits will occur at Months 1 and 3 after discontinuation of lofexidine.