Placebo efficacy was monitored during three separate experiments concerned with the effects of ethanol (0.8 ml/kg body weight) on some aspect of performance: at regular intervals during experimental sessions, blood alcohol concentration was estimated using a breathalyser and subjects completed an intoxication rating. The simple placebo beverages and manipulations employed were similar to those typically described in the literature, and each experiment involved a repeated measures design. Across the three experiments, maximum mean (median) ratings during placebo sessions, expressed as a percentage of those during alcohol sessions of equivalent period, ranged between 10% (0%) and 69% (72%), and the number of subjects in each experiment for whom a placebo effect was considered to have been negligible ranged between 8% and 73%. For each treatment, intoxication ratings were higher during treatment periods which occurred first than during those which occurred second. However, this transfer effect was twice as large for placebo treatments as for alcohol treatments and, whereas in respect of the latter the effect was statistically non-significant, it was highly significant in respect of the former. The occurrence of such asymmetrical transfer of placebo efficacy suggests that repeated measures designs should be used with caution in drug studies in which a convincing placebo treatment is necessary and difficult to devise.
AlcoholPlaceboIntoxicationRepeated measuresAsymmetrical transfer