The concept of placebo/nocebo response and effect.

The placebo response is different from the placebo effect. Whereas the placebo response is the global response to a placebo, including spontaneous remission, regression to the mean, patients' and experimenters' biases, possible effects of cointerventions, as well as psychological factors, the placebo effect includes only the psychological factors, such as the patients' expectations and learning/conditioning. It is like the difference between drug response and drug effect: whereas the former is the global response to a drug, including all the factors described above, the latter represents the specific pharmacodynamic effect. Nocebo responses and nocebo effects go in the opposite direction; these consist of the experience of adverse events when a placebo is administered. The difference between response and effect is crucial for at least two reasons. First, clinical trials are aimed at identifying placebo responses, regardless of the mechanisms that are involved. Second, the identification of the real psychobiological placebo effect aims to understand the underlying mechanisms, that is, what is going on in the patient's brain when he receives a placebo treatment.