Placebo effect

A change associated with treatment context or expectation rather than the specific active component being tested. A comparison with no treatment and appropriate masking is needed to distinguish a placebo effect from spontaneous change or reporting differences.

A belief about the likely consequences of treatment that can influence attention, interpretation, behaviour and symptom experience. Experimental changes in expectation at constant drug exposure can distinguish contextual modulation from differences in administered dose.^[ref]

An intervention transparently described as inert rather than concealed as an active medication. Trials require an appropriate comparator and must address unblinded reporting, explanatory rationale and duration; an effect for one symptomatic outcome is not a universal disease-modifying benefit.^[ref]

The observed change after placebo administration, including possible contextual effects, spontaneous change, regression to the mean and concurrent care. A placebo group's improvement is not by itself an estimate of the causal placebo effect.

Adverse change attributable to a negative treatment context or expectation. It can involve genuine discomfort and physiological modulation as well as reporting and attribution. Its presence does not exclude concurrent disease or a direct adverse drug reaction.^[ref]

A reduction in pain associated with a placebo manipulation. Its influence on responses to others' pain can test shared modulation, without showing that the two experiences have identical neural representations.