Placebo Effect

A placebo (/pləˈsiːboʊ/ plə-SEE-boh; Latin , "I shall please" from , "I please") is a substance or treatment with no active therapeutic effect. A placebo may be given to a person in order to deceive the recipient into thinking that it is an active treatment. In drug testing and medical research, a placebo can be made to resemble an active medication or therapy so that it functions as a control; this is to prevent the recipient(s) and/or others from knowing (with their consent) whether a treatment is active or inactive, as expectations about efficacy can influence results. This phenomenon, in which the recipient perceives an improvement in condition due to personal expectations, rather than the treatment itself, is known as the placebo effect or placebo response. Research about the effect is ongoing.

Placebos are an important methodological tool in medical research. Common placebos include inert tablets (like sugar pills), vehicle infusions, sham surgery, and other procedures based on false information. There is also some evidence that patients who know they are receiving a placebo still report subjective improvement in their condition if they are told that the placebo can make them feel better; however, some of these studies have included placebo substances that may not have truly been inert for subjects who took them, such as a 2010 study involving patients with IBS who were given placebos containing fiber. It has further been observed that use of therapies about which patients are unaware may generally be less effective than using ones that patients are informed about, regardless of whether an or not a placebo is involved.

...
Wikipedia