Pain: Placebo Effects

The placebo effect is the reduction or the disappearance of a symptom when an inert treatment (the placebo) is administered to a subject who is told, and indeed believes and expects, that it is an effective therapy. Most of the knowledge about its mechanisms comes from the field of pain, thus placebo analgesia is currently the most investigated model. However, other systems and apparatuses, such as the motor, immune, and endocrine systems, are emerging as interesting models. Although the placebo effect has so far been considered a nuisance in clinical research when a new treatment has to be tested, it has now become a target of scientific investigation to better understand the physiological and neurobiological mechanisms that link a complex mental activity to different functions of the body. Usually, in clinical research the term placebo effect refers to any improvement in the condition of a group of subjects that has received a placebo treatment. Conversely, the term placebo response refers to the change in an individual caused by a placebo manipulation. However, today these two terms are used interchangeably.

The placebo effect is basically a context effect, whereby the psychosocial context around the patient plays a key role. For example, the therapist's words, the sight of complex machines, and other sensory inputs that tell the patient that a treatment is being performed, all represent important factors in the occurrence of a placebo response. In the case of pain, this psychosocial context is capable of modulating pain perception. This is the reason why the placebo effect is currently a useful model for understanding the complex psychological modulation of pain. This entry describes the identification and mechanisms of the placebo effect, as well as the nocebo effect (a placebo effect in the opposite direction).