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A placebo is a fake or inactive intervention, received by the participants allocated to the control group in a clinical trial, which is indistinguishable form the active intervention received by patients in the experimental group.

One definition is that use of a placebo describes what happens when you do nothing, so that in the context of a clinical trial, for instance, a placebo group could describe the natural history of a disorder without the intervention under test. It is the effect we see when we do nothing.

This rather clumsy phrase is abbreviated to the placebo effect. The trouble is that placebo effect can be taken to mean that any effect seen is caused by placebo. The definition has now changed to mean that doing nothing causes things to happen. The idea that placebo does make things happen is reinforced by much interesting academic work, almost always in contrived situations and with small numbers of people. Extrapolating that to clinical practice is a big step, but one often assumed.

A systematic review and analysis comparing clinical trials in which patients were randomly assigned to placebo or no treatment suggests that the evidence for placebo having an effect is at best weak.