Expectancy in double-blind placebo-controlled trials: an example from alcohol dependence

Background: Double-blind placebo-controlled trials are intended to control for the impact of expectancy on outcomes. Whether they always achieve this is, however, questionable. Methods: Reanalysis of a clinical trial of naltrexone and acamprosate for alcohol dependence investigated this issue further. In this trial, 169 alcohol-dependent patients received naltrexone, acamprosate or placebo for 12 weeks. In addition to being assessed on various indices of alcohol dependence, they were asked whether they believed they received active medication or placebo. Results: While there were no differences in outcomes between treatment groups, those who believed they had been taking active medication consumed fewer alcoholic drinks and reported less alcohol dependence and cravings. That is, irrespective of actual treatment, perceived medication allocation predicted health outcomes. Conclusions: These results highlight the differences between treatment administration in clinical trials and standard medical practice, a discrepancy that may sometimes decrease the validity of these types of trials.