[What place for placebo in clinical trials conducted on psychiatric patients?]

Placebo effect remains a crucial issue in current clinical trials. Most clinical trials rely on the hypothesis of equivalent placebo response rates in both placebo and specific drug arms ("additive model"). But contrary to this dominant and rarely questioned hypothesis, several aspects may influence placebo response. A few recent meta-analyses and reviews have shown evidence for several clinical and methodological factors, which are able to modulate placebo response. In psychiatry research, placebo response has been mainly explored through antidepressant trials. In early clinical trials, drug-placebo differences were initially significant and robust. However, more recent clinical trials have not yielded similar results, and rather show narrowed antidepressant-placebo differences. Several factors may be involved in this absence of comparability: intrinsic properties of new antidepressants, changes in clinical criteria and classifications, symptomatic remission rather than global remission criteria, industrial and institutional constraints. Moreover, results from antidepressant trials (laboratory conditions) remain hardly fully transposable to clinical routine (ecological conditions).