Homeopathy (also homœopathy or homoeopathy; from the Greek ὅμοιος, hómoios, "similar" + πάθος, páthos, "suffering" or "disease") is a form of alternative medicine first defined by Samuel Hahnemann in the 18th century. Homeopathic practitioners contend that an ill person can be treated using a substance that can produce, in a healthy person, symptoms similar to those of the illness. According to homeopaths, serial dilution, with shaking between each dilution, removes the toxic effects of the remedy while the qualities of the substance are retained by the diluent (water, sugar, or alcohol). The end product is often so diluted that materially it is indistinguishable from pure water, sugar or alcohol. Practitioners select treatments according to a patient consultation that explores the physical and psychological state of the patient, both of which are considered important to selecting the remedy.

Claims for efficacy of homeopathic treatment beyond the placebo effect are unsupported by scientific and clinical studies, although meta-analyses of homeopathy, which compare the results of many studies, face difficulty in controlling for the combination of publication bias and the fact that most of these studies suffer from serious shortcomings in their methods. The ideas behind homeopathy are scientifically implausible and "diametrically opposed to modern pharmaceutical knowledge". The lack of convincing scientific evidence supporting its efficacy, Jerry Adler. blank">"No Way to Treat the Dying" - _Newsweek, Feb 4, 2008 and its contradiction of basic scientific principles, have caused homeopathy to be regarded as pseudoscience, or, in the words of a 1998 medical review, as "placebo therapy at best and quackery at worst".

Current usage around the world varies from two percent of people in Britain and the United States using homeopathy in any one year, to 15 percent in India, where homeopathy is now considered part of Indian traditional medicine. Homeopathic remedies are generally considered safe, with rare exceptions; however, homeopaths have been criticised for putting patients at risk by advising them to avoid conventional medicine, such as vaccinations, anti-malarial drugs and antibiotics. In many countries, the laws that govern regulation and testing of conventional drugs often do not apply to homeopathic remedies.