Interest in understanding the underlying mechanisms of the placebo effect has considerably grown during the last few decades. Studies made in this sense have led to a change in the conception of this peculiar phenomenon, and nowadays the placebo effect is viewed as a psychobiological event resulting from the interaction between individual patient factors and factors relating to the physician and the therapeutic environment. Investigation of the placebo effect in disease conditions such as pain or Parkinson’s disease has improved our understanding of its underlying psychological and neurobiological mechanisms. Clinical studies directly designed to investigate the placebo effect have shown placebo to have a more beneficial effect upon diseases evaluated by means of physical or subjective parameters than by means of biochemical parameters. A strong placebo effect has been observed in allergic diseases, where the evaluating parameters tend to be physical or subjective. Biomedical research and the development of new drugs implies an important investment of human and economical resources for conducting clinical trials designed to evaluate the efficacy and safety of new medications. Knowledge of the mechanisms of the placebo effect and how the latter can influence the results of the different efficacy variables in these research studies
appears essential in order to optimize the available resources in application to the development of new drugs.

Key words: Placebo effect. Clinical trial. Allergic rhinitis. Allergy. Allergic rhinoconjunctivitis.