Haney 2015: naltrexone and cannabis in daily smokers

What they tested

Daily cannabis smokers who weren't trying to quit were randomly assigned to naltrexone 50 mg capsules (18 men and 5 women) or placebo capsules (26 men and 2 women) for 16 days. Staff watched people take each capsule, and blood and urine tests confirmed they had.

Before, during and after the medicine period, participants completed 10 lab sessions over 4 to 6 weeks. The sessions measured how often they chose to smoke active or inactive cannabis, how it made them feel, its effects on coordination and its effects on the heart and blood pressure.

What they found

Compared with placebo, naltrexone significantly reduced how much active cannabis people chose to smoke and how much good effect they felt from it. People on placebo had 7.6 times the odds of self-administering active cannabis (95% CI: 1.1-51.8).

The drop in the drug's rewarding effects also showed up in people's cannabis use outside the lab.

Naltrexone had effects of its own: lower ratings of friendliness, less food intake and lower systolic blood pressure, plus more reports of stomach upset and headache. Dropout rates were similar in both groups.

Limits

• It was a lab study of people who weren't trying to quit, so it doesn't show whether naltrexone helps people who want to cut down.
• The groups were small and mostly men (18 men and 5 women on naltrexone).
• The main odds ratio had a very wide confidence interval (1.1 to 51.8).

What it means if you're considering naltrexone

For someone who uses cannabis and is thinking about naltrexone, this study suggests naltrexone can make cannabis feel less rewarding. People on it also reported more stomach upset and headaches. The site's guide on what to avoid on naltrexone cites it for that reason.

It doesn't show that naltrexone treats cannabis use disorder. The authors say clinical trials in people who want to cut down are needed.