Homeopathy (also spelled homoeopathy; 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. The scientific community regards homeopathy as a sham; the American Medical Association considers homeopathy to be quackery,and homeopathic remedies have been criticized as unethical.

Hahnemann believed that the underlying cause of disease were phenomena that he termed miasms, and that homeopathic remedies addressed these. The 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 concentration of homeopathic remedies, which often lack even a single molecule of the diluted substance, has been the basis of questions about the effects of the remedies since the 19th century. 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. This notion has no scientific support. Pharmacological research has found instead that stronger effects of an active ingredient come from higher, not lower doses.

Homeopathic remedies have been the subject of numerous clinical trials. 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, such as cancer or heart disease. The regulation and prevalence of homeopathy vary greatly from country to country.