Concept of true and perceived placebo effects

We often and wrongly equate the response seen in the placebo arm of a clinical trial with the placebo effect. In order to obtain the true placebo effect, other non-specific effects can be identified by including an untreated control group in clinical trials. A review of the literature shows that most authors confuse the perceived placebo effect with the true placebo effect. The true placebo effect is highly variable, depending on several factors that are not fully understood. A distinction between the perceived and the true placebo effects would be helpful in understanding the complex phenomena involved in a placebo response.

Interest in complex issues relating to the placebo effect has recently increased, and in 1994 most major general medical journals focused on the subject.1 2 3 4 Some of the complexities are addressed in this wry definition: placebo is the “most effective medication known to science, subjected to more clinical trials than any other medicament yet nearly always does better than anticipated. The range of susceptible conditions appears to be limitless.”5

This paper sets out to differentiate between the perceived and the true placebo effect. The former is what we commonly assume the placebo effect to be: the response observed in the placebo group of a randomised controlled trial. The true placebo effect equals this response minus other effects that often determine the outcome in all treatment groups of such studies.