A placebo is an inert treatment, be it pharmacological or nonpharmacological, with no specific therapeutic properties for the condition being treated. A placebo effect, i.e., a positive effect on the therapeutic outcome, may follow the administration of such inert treatment. This is different from spontaneous remission or regression to the mean. On the other hand, a nocebo effect encompasses negative therapeutic outcomes. Patients’ expectations have a crucial role in the placebo response (especially with active placebos), and the term placebo effect is often replaced with the term expectation effect . However, although expectation is the most important mechanism that mediates placebo effects, it is not the only one (salience being another; some authors even consider chronic pain a disease of salience): different placebos have different mechanisms of action that could in turn influence outcomes . The problem of why placebo or nocebo effects exist in the first place remains unanswered.
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