The effect of nothing? Time to abandon the concept of placebo
Placebos occupy difficult scientific territory where apparently large effects are attributed to a substance, that is, by definition, inactive. Others have pointed out the flaw in this logic3,4; an agent cannot be both inert and responsible for an effect. Either the agent is active via some particular mechanism or the effect is due to something else. In our view, labelling such effects as “placebo” is unhelpful because it hides the fact that we do not understand what is responsible for the effect. The term complicates and obstructs efforts to understand the effects and mechanisms of medical treatments.
Carvalho et al. clearly suspected a mechanism—expectations. Expectations regarding outcome are indeed a well-documented and testable mechanism of placebos.2 The authors also cite other possibilities in the Introduction section of the article, eg, hope and conditioning. The trial intervention involved education and reassurance about expected outcomes from taking the pills; “the placebo effect can be powerful,” “a positive attitude can be helpful,” and “taking the pills faithfully…is critical”. The intervention group then received the pills along with reinforcement of these messages 1 to 2 weeks later. In other words, what these people received was much more than a placebo pill; they were given a psychological intervention aimed at boosting expectations of a positive outcome. Also of relevance is the fact that the sample was selected based on their belief in mind–body therapy.
We recommend that future studies of these types of interventions explicitly define and test the suspected mechanisms rather than attribute effects to placebo. Moving beyond the placebo black box and investigating specific mechanisms could have real advantages: (1) advancing understanding about pathology and treatment effects by elucidating the real reasons for improvements in outcome; (2) avoiding ethical issues related to giving sick people inactive treatments; and (3) requiring researchers to be explicit about exactly what it is that they are controlling for in randomized trials.
The term “placebo” has enjoyed a long and colourful history, but perhaps its time has passed. Abandoning it may lead to essential improvements in our understanding of the active and inactive components of all interventions. It is time to acknowledge that the concept of having an effect-of-nothing is outdated, unscientific, and holding us back from real progress in the treatment of pain.