[Nocebo effect: the other side of placebo]

Administration of drugs is often followed by beneficial (placebo effects) and harmful (nocebo effects) effects that are not always related to their mechanism of action. Nocebo effects are rather unknown even when may be the source of many adverse reactions which could be erroneously attributed to drug therapy. Some mechanisms have been postulated which might be associated with the development of nocebo effects. Expectancy, learning and classical conditioning are probably important in the psychological domain. The neuropharmacological substrate is much less known yet an opioid peptide-cholecystokinin interaction has been suggested. At the clinical setting, a nocebo effect should be suspected in those patients who present common unspecific symptoms after drug administration and have a tendency to somatize. An early detection of these patients may contribute to the prevention of the nocebo effect.