Naltrexone is contraindicated for patients with hepatic damage or reductions in liver functioning, and is rated in the FDA pregnancy category C. Therefore, blood samples were taken at intake, and months 1, 2, 3, and 5 for liver function (aminotransferase) levels, and pregnancy tests were conducted monthly. Naltrexone was discontinued permanently for one Contingency and one Prescription participant, and temporarily for one Contingency and one Prescription participant due to abnormal aminotransferase levels. Naltrexone was discontinued for one Contingency participant due to pregnancy.