[Understand placebo effect to better treat pain].

PURPOSE: The clinical efficiency of every therapeutic, medicinal or other, contains a part of not specific effect, or placebo effect, of which the frequency of appearance and the importance, in the treatment of pain, can be particularly raised. The practitioners use often, deliberately either not, this effect to modulate their therapeutic efficiency, or in a diagnostic purpose to investigate the mechanism of a pain; our objective is to analyze, in the light of a review of the recent medical literature, what the understanding of the placebo effect brings to the treatment of pain. CURRENT KNOWLEDGE AND KEY POINTS: Neurobiologic mechanisms which sub-aim placebo effect begin to be understood by several studies showing the role of endogen opioid mechanisms. Other studies allow better to understand the psychological determiners of the effect placebo: conditioning mechanisms, and/or cognitive variables, as expectations of the patient or the therapist. FUTURE PROSPECTS AND PROJECTS: At term of this review, we will conclude that the use of a placebo has no value of diagnostic test as for the mechanism of the pain; it is neither necessary nor desirable to implement placebo effect in the daily practice because any therapeutics acts by associating specific and not specific effects. The quality of the relation doctor-patient will allow to mobilize not specific factors susceptible to modulate favorably any therapeutic action. For controlled clinical trials, certain methodologies can be envisaged to by-pass the administration of placebo, reducing so ethical constraints bound to their use.