Placebo is a substance or procedure a patient accepts as medicine or therapy, but which has no specific therapeutic activity. Any therapeutic effect is thought to be based on the power of suggestion.

Placebo controlled trials are those trials where some participants take a placebo as a control and the others take the drug being investigated. Here the placebo is an inactive substance designed to resemble the drug being tested. It is used as a control to rule out any psychological effects which may show during testing. Most well-designed studies include a control group which is unwittingly taking a placebo.

The placebo effect or placebo response is a therapeutic or healing effect of an inert medicine or ineffective therapy, or more generally is the psychosocial aspect of every medical treatment. Sometimes known as a non-specific effect or subject-expectancy effect, the placebo effect (or its counterpart, the nocebo effect), occurs when a patient's symptoms are altered in some way (i.e., alleviated or exacerbated) by a treatment, due to the individual expecting or believing that it will work. The placebo effect occurs when a patient is treated in conjunction with the suggestion from an authority figure or from acquired information that the treatment will aid in healing and the patient's condition improves. This effect has been observed since the early 20th century.

The word placebo has been used in with various meanings; see below.

Etymology

The word placebo is Latin for I will please. It is in Latin text in the Bible (Psalm 114:1–9, Vulgate version), from where it became familiar to the public via the Office of the Dead church service. From that, a singer of placebo became associated with someone who falsely claimed a connection to the deceased to get a share of the funeral meal, and hence a flatterer.

Whenever a placebo is requested in a medical prescription it may imply a statement by the prescribing doctor that "this patient has come to me pleading for a treatment which does not exist or which I cannot or will not supply; I will please him by giving him something ineffectual and claiming that it is effectual." It could also indicate a belief that the effect was due to a subconscious desire of the patient to please the doctor. Since the placebo effect is in the patient not the doctor this may be more self-consistent. Early usage of the term does not indicate why it was chosen.

The word Obecalp, "placebo" spelled backwards was coined by an Australian doctor in 1998 when he recognised the need for a freely available placebo. The word is sometimes used to make the use or prescription of fake medicine less obvious to the patient.

Early use