Novidade na investigação do placebo
: a importância de um braço de estudo vazio

  • Inês Gomes de Cabaço (Student)

Student thesis: Master's Thesis


Introduction: The relief of pain arising from the administration of a treatment that is not directly caused by its pharmacological properties corresponds to what we callthe "placebo effect". Due to the growing scientific interest in the placebo effect and the understanding of the underlying neurobiological mechanisms, the need for a new control group in experimental studies, called "empty arm", emerged. This study aims to assesswhether the introduction of this new arm contributes to an increased ability to predict the placebo effect in a sample of healthy volunteers and, in addition, to conclude on the role of expectations in treatment response. Method: This work, corresponding to a retrospective analysis of a double-blind, crossover experimental study with intra-subject design, includes a sample composed of 51 healthy individuals, randomly distributed by placebo and drug condition. Pain sensitivity measures were applied in two moments - pre and post-test. Information relatedto symptoms and psychological characteristics was collected. Results: It was not possible to induce a placebo effect after the treatment, but when considering the empty arm, specifically in the parameters of the tolerance to mechanical pain and in the thresholds of cold pain, a significant placebo effect was detected. The results also show that high and low negative expectations are associated with higher and lower sensitivity, respectively, after treatment. For positive expectations, lower expectations seem to be associated with lower sensitivity, which may indicate that unrealistic expectations about treatment will have a similar effect to negative expectations. The sequence of treatments also appears to be an important factor in placebo response, with individuals starting with the placebo being more responsive to it.Conclusions: The presence of an empty arm is determinant in the evaluation ofthe placebo effect and expectations (especially negative ones) are an important modulating factor of this same effect and can explain much of the variability in placebo responses found in many studies. In controlled and randomized studies (the so-called RCT's) with three arms, the sequence of treatments must be taken into account, being avariable deeply influenced by the expectations developed. The scarcity of research on the placebo effect with the methodology adopted here and on the healthy population justifies the investment in this type of study, allowing for the monitoring of changes in pain levels that occur in these individuals.
Date of Award15 Apr 2021
Original languagePortuguese
Awarding Institution
  • Universidade Católica Portuguesa
SupervisorRita Canaipa (Supervisor) & Roi Treister (Co-Supervisor)


  • Placebo response
  • Empty arm
  • Pain assessment
  • Expectations
  • Randomized and cross-over study


  • Mestrado em Neuropsicologia

Cite this