Nineteen randomised controlled trials were included (n=2,691: 1,454 received naltrexone and 1,237 received placebo).
Twelve studies were single-site and seven were multi-site.
The average effect size for the percentage of drinking days was significantly greater than zero with naltrexone for both single site studies (d=0.33, 95% CI: 0.10, 0.56) and multicentre studies (d=0.17, 95% CI: 0.03, 0.36). There was no significant difference in the effect sizes between single-site studies and multicentre studies. The effect of the year of publication showed a non significant negative trend, with earlier studies showing larger effect sizes. The inclusion of both study type and year of publication as predictors of effect size reduced the magnitude of both effects.
The effect size estimates for the percentage of participants who relapsed to heavy drinking was significantly greater than zero with naltrexone for both single-site studies (d=0.41, 95% CI: 0.25, 0.55) and multicentre studies (d=0.17, 95% CI: 0.04, 0.29). The effect size estimates from multicentre studies were significantly smaller than the average effect size for single-site studies. The effect of the year of publication was also significant, with smaller effect sizes in more recent studies. The inclusion of both the type of study and the year of publication resulted in neither factor being a significant predictor of effect size.