Homeopathy (also spelled homoeopathy or homœopathy) is a form of alternative medicine in which practitioners claim to treat patients using highly diluted preparations that are believed to cause healthy people to exhibit symptoms that are similar to those exhibited by the patient. The collective weight of scientific evidence has found homeopathy to be no more effective than a placebo.

In the context of homeopathy, the term remedy is used to refer to a substance which has been prepared with a particular procedure and intended for patient use; it is not to be confused with the generally accepted use of the word, which means "a medicine or therapy that cures disease or relieves pain".

The basic principle of homeopathy, known as the "law of similars", is "let like be cured by like". The term was coined from the Greek hómoios- ?μοιος- "like-" + páthos π?θος "suffering" The principle was first stated by German physician Samuel Hahnemann in 1796, and is not a true law of nature. Homeopathic remedies are prepared by serial dilution with shaking by forceful striking on an elastic body, which homeopaths term succussion. Each dilution followed by succussion is assumed to increase the effectiveness. Homeopaths call this process potentization. Dilution often continues until none of the original substance remains. Apart from the symptoms, homeopaths examine aspects of the patient's physical and psychological state, then homeopathic reference books known as repertories are consulted, and a remedy is selected based on the totality of symptoms.

While some individual studies have positive results, systematic reviews of published trials fail to demonstrate efficacy. Furthermore, higher quality trials tend to report results that are less positive, and most positive studies have not been replicated or show methodological problems that prevent them from being considered unambiguous evidence of homeopathy's efficacy.

Depending on the dilution, homeopathic remedies may not contain any pharmacologically active molecules, and for such remedies to have pharmacological effect would violate fundamental principles of science. Modern homeopaths have proposed that water has a memory that allows homeopathic preparations to work without any of the original substance; however, there are no verified observations nor scientifically plausible physical mechanisms for such a phenomenon. The lack of convincing scientific evidence to support homeopathy's efficacy and its use of remedies lacking active ingredients have caused homeopathy to be described as pseudoscience, quackery, and a "cruel deception".

Oral homeopathic remedies are safe at the high dilutions recommended by Hahnemann, since they likely contain no molecules of the original substance, but they may not be safe at lower dilutions.[29] Homeopathy has been criticized for putting patients at risk due to advice against conventional medicine such as vaccinations,[30] anti-malarial drugs, and antibiotics.

The regulation and prevalence of homeopathy is highly variable from country to country. There are no specific legal regulations concerning its use in some countries, while in others, licenses or degrees in conventional medicine from accredited universities are required. In several countries, homeopathy is covered by the national insurance to different extents, while in some it is fully integrated into the national healthcare system. In many countries, the laws that govern the regulation and testing of conventional drugs do not apply to homeopathic remedies.