Effects of long-term therapy with naltrexone on body weight in obesity

The endogenous opiate system is thought to be associated with the regulation of food intake and body weight. Opiate antagonists decrease food intake in animals, but there are no controlled studies in obese man to evaluate body weight response to naltrexone. Sixty obese people were randomized into three groups and given 0, 50, or 100 mg of the opiate antagonist naltrexone for 8 weeks in an outpatient, double-blind study. Weight loss was not significant in either the 50 or 100 mg groups as compared with placebo. However, when broken down by sex, women had a significant (P less than 0.05) weight loss of 1.7 kg, while men did not lose weight. Side effects were modest, but six subjects had one or more abnormal liver function test results; in one subject these abnormalities appeared to be clinically significant. The effects of naltrexone on weight loss were less than expected in light of prior animal studies, but further studies with a wider dose range of naltrexone may be indicated.