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.