Placebo - Mechanism of The Effect

Mechanism of The Effect

The phenomenon of a patient's perceived medical improvement following treatment with an inert substance is called the placebo effect. The placebo effect is highly variable in its magnitude and reliability and is typically strongest in measures of subjective symptoms (e.g., pain) and typically weak-to-nonexistent in objective measures of health points (e.g., blood pressure, infection clearance).

A 2001 meta-analysis of clinical trials with placebo groups and no-treatment groups found no evidence for a placebo effect on objectively measured outcomes and possible small benefits in studies with continuous subjective outcomes (particularly pain). A 2004 follow-up analysis found similar results and increased evidence of bias in smaller trials that calls into question the apparent placebo effect on subjective outcomes.

Because the placebo response is simply the patient response that cannot be attributed to an investigational intervention, there are multiple possible components of a measured placebo effect. These components having varying relevance depending on study design and the types of observations. While there is some evidence that placebo interventions can alter levels of hormones or endogenous opioids, other prominent components include expectancy effects, regression to the mean, and flawed research methodologies.