We obtained data on 23 trials of naltrexone and 22 trials of acamprosate from Cochrane reviews. We extracted data for 14 study characteristics and 3 dependent variables (odds ratio; percent abstinent in placebo; medication conditions). We used general linear models to determine which study characteristics explained the variability among outcomes after controlling for medication characteristics.

Results

Study characteristics accounted for 45% of the variance in odds ratio when trials of different medications were combined, 19% among acamprosate, and 48% among naltrexone trials above and beyond medication characteristics. When medications were combined, greater odds ratios were predicted by having more dropouts in the placebo than medication conditions, an earlier publication year, and not receiving industry funding. Whether this was due to effects on placebo or medication conditions was unclear. Among acamprosate trials, smaller sample sizes predicted greater odds ratios, which appeared to be due to more abstinence in medication conditions. Among naltrexone trials, greater odds ratios were predicted by having more dropouts in the placebo than medication conditions and a greater probability of randomizing participants to treatment. This appeared to be due to less abstinence in placebo conditions.