Placebo effect

Improvement in symptoms from a treatment with no active therapeutic effect, driven by expectation, conditioning, and the context of care.

Expectation: A belief about what will happen that shapes perception, physiology, and behavior before outcomes arrive. Expected relief recruits the same neural machinery as active drugs, especially in pain.
Placebo: Latin for 'I shall please'; by medieval times, a term for professional mourners and flatterers. The name of the effect came from this older word for pleasing rather than healing.
Beecher's 1955 'Powerful Placebo': Henry Beecher's synthesis of 15 trials concluding placebo relief in roughly one-third of patients. The founding paper that legitimized placebo research in modern medicine.
Randomized controlled trial: The experiment design assigning treatments by chance to isolate a treatment's specific effect. Placebo controls are its defining feature; the effect created the design's need.
Regression to the mean: The statistical tendency of extreme states, like worst-ever symptoms, to return toward average over time. Much 'placebo improvement' in trials is this artifact, per critics of the effect.
Conditioning: Learning in which a repeated association between a cue and an outcome eventually elicits the response itself. Prior drug exposure lets placebo cues trigger drug-like responses without the drug.
Hôpital Beaujon placebo trials: Paris physician Émile Littré's 1834 proposal to test treatments against inert dummy pills. An early plan to use inert controls as a scientific tool.
Benedetti's naloxone experiment: Fabrizio Benedetti's demonstration that naloxone abolishes placebo analgesia. Directly identified endogenous opioids as the substance behind placebo pain relief.
Informed consent: The ethical requirement that patients understand a treatment before agreeing to it. Deceptive placebo use collides with it; open-label placebo partially resolves the clash.
Natural course of illness: How a disease would unfold without any treatment at all. Symptoms that improve on their own get misattributed to inert pills.

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