Homeopathy i/ˌhoʊmiˈɒpəθi/ (also spelled homoeopathy or homœopathy; from the Greek hómoios- ὅμοιος- "like-" + páthos πάθος "suffering") is a system of alternative medicine originated in 1796 by Samuel Hahnemann, based on his doctrine of similia similibus curentur ("like cures like"), according to which a substance that causes the symptoms of a disease in healthy people will cure similar symptoms in sick people. Scientific research has found homeopathic remedies ineffective and their postulated mechanisms of action implausible. Within the medical community homeopathy is considered to be quackery.
Homeopathic remedies are prepared by repeatedly diluting a chosen substance in alcohol or distilled water, followed by forceful striking on an elastic body, called succussion. Each dilution followed by succussion is said to increase the remedy's potency. Dilution usually continues well past the point where none of the original substance remains. Homeopaths select remedies by consulting reference books known as repertories, considering the totality of the patient's symptoms as well as the patient's personal traits, physical and psychological state, and life history.
The low concentrations of homeopathic remedies, often lacking even a single molecule of the diluted substance, lead to an objection that has dogged homeopathy since the 19th century: how, then, can the substance have any effect? Modern advocates of homeopathy have suggested that "water has a memory"—that during mixing and succussion, the substance leaves an enduring effect on the water, perhaps a "vibration", and this produces an effect on the patient. However, nothing like water memory has ever been found in chemistry or physics. Pharmacological research has found, contrary to homeopathy, that stronger effects of an active ingredient come from higher doses, not lower doses.
Homeopathic remedies have been the subject of numerous clinical trials, which test the possibility that they may be effective through some mechanism unknown to science. Taken together, these trials showed at best no effect beyond placebo, at worst that homeopathy could be actively harmful. Although some trials produced positive results, systematic reviews revealed that this was because of chance, flawed research methods, and reporting bias. The proposed mechanisms for homeopathy are precluded by the laws of physics from having any effect. Patients who choose to use homeopathy rather than evidence based medicine risk missing timely diagnosis and effective treatment of serious conditions. The regulation and prevalence of homeopathy vary greatly from country to country.