Atkinson 1985: naltrexone alone didn't cause weight loss

What they tested

Sixty people with obesity were randomly put into three groups taking 0 (a placebo, or dummy pill), 50 or 100 mg of naltrexone for 8 weeks. Neither they nor the researchers knew who was taking which.

The idea came from animal studies, where opioid blockers cut how much animals ate. The authors said there had been no controlled studies of naltrexone and body weight in people with obesity.

What they found

Weight loss in the 50 mg and 100 mg groups was not significant compared with placebo.

Broken down by sex, women had a significant weight loss of 1.7 kg (P less than 0.05), while men did not lose weight.

Side effects were modest, but six people had one or more abnormal liver test results. In one person the abnormal results appeared to be clinically significant.

Limits

• It was small and short, with 60 people over 8 weeks.
• The result in women comes from splitting the groups by sex after the fact, which is weaker evidence than the main comparison.
• It tested naltrexone alone, not the naltrexone-bupropion combination sold for weight management.

What it means if you're considering naltrexone

On its own, naltrexone did little for weight in this trial, and the authors said the effect was smaller than animal studies had suggested. The weight-management medicine that contains naltrexone, Contrave, pairs it with bupropion, and this trial says nothing about that combination.