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A placebo (Latin: I shall please)[2] is a sham or simulated medical intervention that can produce a (perceived or actual) improvement, called a placebo effect.

In medical research, placebos depend on the use of controlled and measured deception. Common placebos are inert tablets, sham surgery,[3] and other procedures based on false information.[1] In one common placebo procedure, a patient is given an inert pill, told that it may improve his/her condition, but not told that it is in fact inert. Such an intervention may cause the patient to believe the treatment will change his/her condition; and this belief may produce a subjective perception of a therapeutic effect, causing the patient to feel their condition has improved. This phenomenon is known as the placebo effect.

Placebos are widely used in medical research and medicine,[4] and the placebo effect is a pervasive phenomenon;[4] in fact, it is part of the response to any active medical intervention.[5] The placebo effect points to the importance of perception and the brain's role in physical health. However, when used as treatment in clinical medicine (as opposed to laboratory research), the deception involved in the use of placebos creates tension between the Hippocratic Oath and the honesty of the doctor-patient relationship.[6] The United Kingdom Parliamentary Committee on Science and Technology has stated that: "...prescribing placebos... usually relies on some degree of patient deception" and "prescribing pure placebos is bad medicine. Their effect is unreliable and unpredictable and cannot form the sole basis of any treatment on the NHS."[3]

Since the publication of Henry K. Beecher's The Powerful Placebo in 1955 the phenomenon has been considered to have clinically important effects.[7] This view was notably challenged when in 2001 a systematic review of clinical trials concluded that there was no evidence of clinically important effects, except perhaps in the treatment of pain and continuous subjective outcomes.[7] The article received a flurry of criticism,[8] but the authors later published a Cochrane review with similar conclusions (updated as of 2010).[9] Most studies have attributed the difference from baseline till the end of the trial to a placebo effect, but the reviewers examined studies which had both placebo and untreated groups in order to distinguish the placebo effect from the natural progression of the disease.[7]

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