The placebo effect is an effect of a medical treatment that is attributable to an expectation that the treatment will have an effect, i.e., the effect is due to the power of suggestion. Thus the placebo effect can have a true physiologic role as a component of the effect of a treatment.
Formally, it is "an effect usually, but not necessarily, beneficial that is attributable to an expectation that the regimen will have an effect, i.e., the effect is due to the power of suggestion." This means that an active drug or intervention has additional effects, or even the suppression of side effects, due to suggestion by the clinician.
"The placebo effect is nothing new, nor are attempts to enhance its effect unconventional. In fact the history of conventional medicine has largely been the history of the placebo effect... Most medicines used by doctors up until the 20th Century are now known to be inert, but they were often of exotic origin and thus were often perceived as having magical properties. Even today part of the conventional doctor's armoury may include inert capsules and sugar pills. In fact one study showed that 80 per cent of US hospital clinicians admitted to the occasional use of placebo medicines in routine clinical practice....Many studies have been conducted where placebo treatments have been compared to no-treatment controls. Evidence from a wide range of studies indicates that placebo therapies in the context of conventional medicine can provide some relief from a huge range of conditions including allergies, angina, asthma, some forms of cancer, cerebral infarction, depression, diabetes, epilepsy, multiple sclerosis, ulcers and warts. Placebo responses have also been found to vary enormously —from 0 - 100 per cent — even for the same condition" (House of Lords Select Committee on Science and Technology, Sixth Report, "Complementary and Alternative Medicine") 
The word placebo was first used in conjunction with medical treatments by William Cullen in the 18th century. Cullen, the leading physician of the day, used it to describe treatments that he gave with the intention of reassuring the patient rather than with any rational expectation that they might be effective through objective physiological mechanisms. For Cullen, a placebo was a conventional remedy given at a dose lower than he expected would have any physiological effect ("I make it a rule even in employing placebos to give what would have a tendency to be of use to the patient.") He describes a patient for whom he prescribed mustard, that he thought "might be useful in paralytic affections"; but importantly he declared that "I own that I did not trust much to it, but I gave it because it is necessary to give a medicine, and as what I call a placebo." . Cullen was an important Scottish Enlightenment thinker on the functions of the nervous system and in medicinal chemistry. Along with his contemporaries in Edinburgh, including the economist Adam Smith and the philosopher David Hume, Cullen developed a theory of 'sympathy', a kind of 'vitalism vital force' that coordinated the functions of the human body, and which transmitted sensation to target organs. As a theorist of ‘sympathy’ and ‘vitalism’, he propounded what would now be described as a psychosomatic theory of illness.
The Power of the Placebo
In 1784, the King of France set up a Commission to investigate the claims of Franz Mesmer and his followers, who claimed remarkable curative effects of "animal magnetism". The commission, headed by Benjamin Franklin and including Lavoisier, by a series of careful tests exposed the effects of animal magnetism to be effects of suggestion alone.
" The commissioners have ascertained that the animal magnetic fluid is not perceptible by any of the senses ; that it has no action, either on themselves or on the patients subjected to it. They are convinced that pressure and contact effect changes which are rarely favourable to the animal system and which injuriously affect the imagination. Finally, they have demonstrated by decisive experiments that imagination apart from magnetism, produces convulsions, and that magnetism without imagination produces nothing. They have come to the unanimous conclusion with respect to the existence and utility of magnetism, that there is nothing to prove the existence of the animal magnetic fluid; that this fluid, since it is non-existent, has no beneficial effect; that the violent effects observed in patients under public treatment are due to contact, to the excitement of the imagination, and to the mechanical imitation which involuntarily impels us to repeat that which strikes our senses."
(From the report of the Commissioners).
For very many conditions, it is now clear that the placebo effect can be very powerful indeed. In 1955, H.K. Beecher published The Powerful Placebo in which he concluded that, in 26 studies that he analyzed, an average of 32 percent of patients responded to placebo. In the 1960s, many studies showed physiological effects of dummy pills--they tended to speed up pulse rate, increase blood pressure, and improve reaction speeds, for example, when participants were told they had taken a stimulant, and had the opposite effects when participants were told they had taken a sedative.
Mechanisms underlying the placebo effect
In the first decade of the 21st century, there was an explosion of research into the placebo effect, as brain imaging technology made it possible to visualise, in human volunteers, the brain activity underlying the placebo effect. Not so very long ago, most peripheral organs of the body were assumed to be largely autonomously functioning; now we know that virtually every organ and tissue is extensively innervated - even fat stores are directly innervated by neural pathways descending from the hypothalamus. The capacity of the CNS to influence selectively peripheral function is thus apparently considerable. This is not under conscious control - but autonomic reflexes can be conditioned - so there is an element of subconscious control. Control of the immune system - neuroimmunology - is a rapidly expanding field; the immune system is regulated in part by the hypothalamo-pituitary adrenal axis (the so-called "stress axis or HPA axis), and in part by other mechanisms that are still not clearly understood.
The placebo effect may be partly determined by genetics.
Thus directly or indirectly the brain controls most of the organs and systems of the body in ways that might potentially influence the course of disease. Does the placebo effect involve subconscious harnessing of these pathways?
Use in Medicine
Many modern physicians do not consider that the deliberate use of placebos is ethical, ad they consider it unethical to knowingly mislead their patients.
- Anonymous (2015), Placebo effect (English). Medical Subject Headings. U.S. National Library of Medicine.
- House of Lords Select Committee on Science and Technology, Sixth Report, 2000"Complementary and Alternative Medicine"; Chapter 3: Patient satisfaction, the role of the therapist and the placebo response
- Kerr CE, Milne I, Kaptchuk TJ [ http://www.jameslindlibrary.org/trial_records/17th_18th_Century/cullen/cullen-commentary.html William Cullen and a missing mind-body link in the early history of placebos ] Commentary on: Cullen W (1772). Clinical lectures. Edinburgh, Feb-April, 218-9.In: The James Lind Library Accessed Sunday 11 January 2009
- Beecher HK, cited by Tamar Nordenberg, "The Healing Power of Placebos", FDA Consumer Magazine", January-February 2000, 
- Hall KT, Lembo AJ, Kirsch I, Ziogas DC, Douaiher J, Jensen KB et al. (2012). "Catechol-O-Methyltransferase val158met Polymorphism Predicts Placebo Effect in Irritable Bowel Syndrome.". PLoS One 7 (10): e48135. DOI:10.1371/journal.pone.0048135. PMID 23110189. PMC PMC3479140. Research Blogging.
- AMA: Ethics Council's Stance on Placebo Therapy Stirs Unease MedPage Today 16th June 2006