Placebo Effect

The placebo effect is a change produced by the administration of a substance or physical procedure that is not produced by the physical properties of that substance or procedure. It is, instead, an effect produced by the psychological effects of administering the treatment. A placebo is a substance or procedure that cannot produce a particular effect by virtue of its physical properties. This entry describes the history of the placebo concept, its use in clinical trials, factors affecting the magnitude of the placebo effect, and theoretical accounts of how placebo effects are produced.

The word placebo comes from the Latin word placebo meaning “I shall please,” and for centuries, it was assumed that placebos could placate troubled patients but not produce any real changes. It was not until the mid 20th century that researchers began to appreciate that placebos might produce changes in symptoms. Once that realization became widespread, placebos became commonplace in the process of approving new drugs. Randomized controlled trials (RCTs) soon became the gold standard for testing new medications.

In an RCT, subjects are randomly assigned to at least two groups, an active treatment group and a placebo group. Some trials may include three or more groups. For example, two different drugs may be compared to each other and to a placebo, or subjects might be randomized to receive different doses of the same drug. Assignment to treatment condition is double-blind, which means that neither the physician nor the patient is told to which group the patient has been assigned. The placebo is made so as to be indistinguishable from the real drug. It is the same color, size, and shape. The idea is to keep [Page 604]all psychological variables constant between the active and placebo treatment conditions so that any differences in the effects obtained can be unequivocally ascribed to the active physical properties of the treatment.

Usually, placebos are physically inert, but active placebos are sometimes used. An active placebo is an active substance that can produce side effects but that should have no physical effect on the condition being treated. The purpose of using an active placebo is to prevent patients from breaking blind. Breaking blind occurs when patients are able to figure out which group they have been assigned to, perhaps because of the side effects that the real drug produces.