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[What do we know about the mechanism of action of the placebo?].

The placebo is a substance or a procedure capable to induce a subjective or objective healing effect, but which efficacy or mechanism of action could not be demonstrated by the conventional scientific procedures. The main objective of this review is to synthesize the theories about the way the placebo acts. Conditioning, expectancy and desire theories, as well as the possible symbolic level in which placebo acts are reviewed. Some recent investigations showed that nerobiological responses elicited by placebo administration mimic those induced by it corresponding active drug. Finally, following evidences from development research, we suggest, that early expierences provide individuals with a "correction program of physiological functions" which could be eventually activated by contextual clues, as placebo.