[Influence of placebo effect on planning and analysis of clinical trials].

Placebos, seen as any pharmacological procedure without a specific effect, have been widely used in medical practice. In the fifties, when clinical trial development started, placebos assume a new dimension, as fundamental tools in clinical trial design. The aim of this review was to evaluate and describe methodological aspects related to placebo use in clinical trials. Despite the importance attributed to placebos, particularly to maintain blinding, it is not always easy to simulate an active procedure. On the other hand, the ethical aspects of placebo utilization in some clinical and experimental circumstances are becoming more and more complex. These two aspects are commented. It is emphasized that the use of a placebo, by itself, may influence the results of a clinical study. We analyze the factors that determine the placebo effect but also how placebos and the placebo effect affect final results.