Homeopathy (also spelled homœopathy or homoeopathy) from the Greek words όμοιος, hómoios (similar) and πάθος, páthos (suffering), is a system of alternative medicine that treats "like with like", using remedies that it is claimed would, in healthy individuals, produce similar symptoms to those it would treat in an ill patient. Practitioners believe that the potency of a remedy can be increased by systematically diluting the dosage, along with succussion or shaking, to a point where the original ingredient is not present.

Although homeopathy is reported to be rapidly growing in popularity, it is controversial and does not satisfy the scientific standards of evidence-based medicine.

The term "homeopathy" was coined by the Saxon physician Samuel Hahnemann (1755–1843) and first published in 1796.

Homeopathy is founded on the 'Law of Similars', first expressed by Hahnemann in the exhortation similia similibus curentur or 'let likes cure likes'. The law of similars is based on Hahnemann's conclusion that a given constellation of symptoms ellicited by a given homepathic remedy in a group of healthy individuals will cure an ill individual exhibiting a similar constellations of symptoms. Symptom patterns associated with various remedies are determined by 'provings', in which healthy volunteers are given remedies in homeopathic form, and the physical, mental and spiritual symptoms they develop are recorded and complied by observers. Homeopathic practitioners rely on two types of reference in prescribing. The Homeopathic Materia Medicae are organized by remedy, and describe symptom patterns associated with individual remedies. The Homeopathic repertory are organized by symptom, and list individual remedies associated with various symptoms.

Homeopathic remedies are prepared by dilution of a substence with succussion, or shaking, between dilutions. The remedies in homeopathy are often so dilute that they are statistically unlikely to contain any molecules of the original substance. At first, Hahnemann tested substances commonly used as medicines in his time and poisions in homeopathic provings. He recorded his findings in his Materia Medica Pura. Kent's Lectures on Homoeopathic Materia Medica (1905) lists 217 remedies, and new substances are being added continually to contemporary versions. Homeopathy uses many animal, plant, mineral, and chemical substances of natural or synthetic origin. Examples include Natrum muriaticum (sodium chloride or table salt), lachesis muta (the venom of the bushmaster snake), Opium, and Thyroidinum (thyroid hormone). Other homeopathic remedies, ('isopathic' remedies) involve dilutions of the agent or the product of the disease. Rabies nosode, for example, is made by potentizing the saliva of a rabid dog. Some modern homeopaths are exploring the use of more esoteric substances, known as imponderables because they do not originate from a material substance but from electromagnetic or electrical energy presumed to have been captured by direct exposure (X-ray, Sol (sunlight), Positronium, and Electricitas (electricity)) or through the use of a telescope (Polaris). Recent ventures by homeopaths into esoteric substances include Tempesta (thunderstorm), and Berlin wall.

Today, about 3000 remedies are used in homeopathy; about 300 are based on comprehensive Materia Medica information, about 1500 on relatively fragmentary knowledge, and the rest are used experimentally in difficult clinical situations based on the law of similars, either without knowledge of their homeopathic properties or through knowledge independent of the law of similars. Examples include: the use of an isopathic (disease causing) agent as a first prescription in a 'stuck' case, when the beginning of disease coincides with a specific event such as vaccination; the use of a chemically related substance when a remedy fails yet seems well-indicated; and more recently, the use of substances based on their natural classification (the periodic table or biological taxonomy). This last approach is considered to be promising by some in the homeopathic community, because it allows for grouping remedies and classifying the ever-burgeoning Materia Medica, but is rejected by many purists because it involves speculation about remedy action without proper provings.

There are many methods for determining the most-similar remedy (the simillimum), and homeopaths often disagree about the diagnosis. This is due in part to the complexity of the idea of 'totality of symptoms'; homeopaths do not use 'all' symptoms, but decide which are the most characteristic; this evaluation is the aspect of diagnosis requiring the most knowledge and experience. Finally, the remedy picture in the Materia Medica is always more comprehensive than the symptomatology that one individual ever exhibits. These factors mean that a homeopathic diagnosis remains presumptive until it is verified by testing the effect of the remedy on the patient.

The law of similars is not a 'scientific' law in the sense that it is not built on a hypothesis that can be falsified scientifically; a failure to cure homeopathically can always be attributed to incorrect selection of a remedy.

The most characteristic—and controversial—principle of homeopathy is that the potency of a remedy can be enhanced (and the side-effects diminished) by dilution, in a procedure known as dynamization or potentization. Liquids are successively diluted (with water, or alcohol for water-insoluble materials) and shaken by ten hard strikes against an elastic body (succussion). Insoluble solids are diluted by grinding them with lactose (trituration). Higher dilutions are considered to be stronger 'deep-acting' remedies.

The dilution factor at each stage is traditionally 1:10 ('D' or 'X' potencies) or 1:100 ('C' potencies). Hahnemann advocated 30C dilutions for most purposes, i.e. dilution by a factor of 10030 = 1060. As Avogadro's number is only 6.022 × 1023 particles/mole, the chance of any molecule of the original substance being present in a 15C solution is small, and it is extremely unlikely that one molecule would be present in a 30C solution. For perspective on these numbers, there are under 1051 atoms in the Earth, and on the order of 1032 molecules of water in an Olympic size swimming pool; to expect to get one molecule of a 15C solution, one would need to take 1% of the volume of such a pool, or roughly 25 metric tons of water, comparable to the weight of a fully-loaded 18-wheeler. Thus homeopathic remedies that have a high "potency," with overwhelming probability, contain only water, but this water is believed by practitioners of homeopathy to retain some 'essential property' of the substance once present. A key criticism is that any water will, at some time in its history, have been in contact with many different substances. Thus, any drink may be considered to be an extreme dilution of almost any agent you care to mention. Thus, critics argue that almost everyone is almost always receiving homeopathic treatment for almost every condition. Proponents of homeopathy respond that the methodical dilution of a particular substance, beginning with a 10% solution and working downward, is different; exactly why this is different is not clear. Thus modern medicine rejects the possibility of highly diluted preparations having any medicinal action, but attributes claimed effects to the Placebo Effect and/or the Forer effect.

Later homeopaths advocated very high potencies, which could not be made by traditional methods, but required succussion without dilution (Jenichen), higher dilution factors (LM potencies are diluted by a factor of 50,000), or machines which integrate dilution and succussion into a continuous process (Korsakoff). The practitioner's choice of what potency is appropriate is subjective; it involves his or her opinion of how "deep-seated" the disease is; whether it is primarily physical or more mental/emotional; the patient's sensitivity based on the practitioner's intuitive assessment or previous reactions to remedies; and the desired dosing regimen (e.g. low potency repeated often, vs high potency repeated seldom). Generally, French and German homeopaths use lower potencies than their American counterparts. Most homeopaths believe that the choice of potency is secondary to the choice of remedy: i.e. that a well-chosen remedy will act in a variety of potencies, but an approximately matched remedy might act only in certain potencies.

By 1816, Hahnemann was concerned at the failure of his homeopathic remedies to produce lasting cures for chronic diseases. He found that "...the non-venereal chronic diseases, after being time and again removed homoeopathically … always returned in a more or less varied form and with new symptoms." To explain this, Hahnemann introduced the miasmatic theory, that three fundamental "miasms" are behind all the chronic diseases of mankind: syphilis, sycosis, and psora. The miasm of psora, he concluded, was behind most of the chronic diseases known to medicine. Miasma, from the Greek for 'stain', was an old medical concept, used for "pestiferous exhalations". The sense of this is indicated by Hahnemann's Note 2 to §11 of the Organon: "...a child with small-pox or measles communicates to a near, untouched healthy child in an invisible manner (dynamically) the small-pox or measles, … in the same way as the magnet communicated to the near needle the magnetic property..."

According to Hahneman, miasmatic infection causes local symptoms, usually in the skin. If these are suppressed by external medication, the disease goes deeper, and manifests itself as organ pathologies. In §80 of the Organon he asserted psora to be the cause of such diseases as epilepsy, cyphosis, cancer, jaundice, deafness, and cataract.

Even in his own time, many followers of Hahneman, including Hering, made almost no reference to Hahnemann’s concept of chronic diseases. Today, some homeopathic practitioners  accept that Hahnemann’s theory does not hold up in light of current knowledge in immunology, genetics, microbiology and pathology, and recognise that Hahnemann neglected to identify genetic, congenital, metabolic, nutritional, and degenerative diseases, and failed to differentiate the multitude of different infectious diseases. However, they believe that some elements of his theory are valid. For instance, they believe that the fundamental cause of disease is constitutional (i.e. the susceptibility to becoming ill), and that it is contrary to good health to suppress symptoms, especially skin eruptions and discharges. They also accept Hahneman's concept of latent psora, the early signs of an organism’s imbalance that indicate that treatment is needed to prevent the development of more advanced disease.

Theory of disease

In Hahnemann's day, the conventional theory of disease was based on the four humours. Mainstream medicine focused on restoring the balance in the humours, either by attempting to remove an excess of a humour (by such methods as bloodletting and purging, laxatives, enemas and nauseous substances that made patients vomit) or by suppressing symptoms associated with the humours causing trouble, such as by lowering the body temperature of patients who were feverish. By contrast, Hahnemann promoted a view of 'spiritual factors' as the root cause of all disease. Some later homeopaths, in particular James Tyler Kent, put even more emphasis on spiritual factors.

"...for it goes to the very primitive wrong of the human race, the very first sickness of the human race that is the spiritual sickness... which in turn laid the foundation for other diseases."

Vitalism was a part of mainstream science in the 18th century. In the twentieth century, medicine discarded vitalism in favour of the germ theory of disease, following the work of Louis Pasteur, Alexander Fleming, Joseph Lister and many others. Modern medicine sees bacteria and viruses as the causes of many diseases, but Kent, and some modern homeopaths regard them as effects, not causes, of disease. Others have adapted to the views of modern medicine by referring to disturbances in, and stimulation of, the immune system, rather than the vital force.

Hahnemann developed homeopathy after coming upon the idea that 'like cures like' while translating a work on malaria. On reading that Cinchona bark (which contains quinine) was effective because it was bitter, Hahnemann felt this implausible because other substances were as bitter but had no therapeutic value. To understand the effects of Cinchona bark, he decided to take it himself, and saw that his reactions were similar to the symptoms of the disease it was used to treat. At least one writer has suggested that Hahnemann was hypersensitive to quinine, and that he may have had an allergic reaction.

For Hahnemann, the whole body and spirit was the focus of therapy, not just the localised disease. Hahnemann spent a lot of time with his patients, asking them not only about their symptoms or illness, but also about their daily lives. This gentle approach contrasted with the violent forms of heroic medicine common at the time, which included techniques such as bleeding as a matter of course.

Homeopathy came to the USA in 1825 and rapidly gained popularity, partly because the excesses of conventional medicine were extreme there, and partly due to the efforts of Constantine Hering. Homeopathy reached a peak of popularity in 1865–1885 and thereafter declined due to a combination of the recognition by the establishment of the dangers of large doses of drugs and bleeding, and dissent between different schools of homeopathy.

Nearly as important as Hahnemann to the development of homeopathy was James Tyler Kent (1849 – 1921). Kent's influence in the USA was limited, but in the UK, his ideas became the homeopathic orthodoxy by the end of the First World War. His most important contribution may be his repertory, which is still used today. Kent's approach was authoritarian, emphasizing the metaphysical and clinical aspects of Hahnemann's teachings, in particular
insistence on the doctrines of miasm and vitalism;
emphasis on psychological symptoms (as opposed to physical pathology) in prescribing; and
regular use of very high potencies.

Kent believed that illness had spiritual causes:

"You cannot divorce medicine and theology. Man exists all the way down from his innermost spiritual, to his outermost natural." (Kent, 1926)

and in the USA, homeopathy came to be associated closely with Swedenborgianism, (the Christian mystical sect of Emanuel Swedenborg, who founded the New Jerusalem Church)[4]. All prominent American homoeopaths in the nineteenth century, from Hering to Kent, were members of the New Jerusalem Church; and the members of the Church were mostly supporters and followers of homoeopathy. In Russia, homoeopathy was similarly closely connected with the Russian Orthodox Church, and the Missionary School of Medicine, founded in England in 1903, was closely associated with the Faculty of Homoeopathy in London in the early 1900's.

Today, the ease with which large databases can be manipulated has profoundly changed the way homeopathy is practised. Today, many homeopaths use computers to sift through thousands of provings and case studies. Because information about lesser-known remedies is more accessible, it is now more common for homeopaths to prescribe them, which has led to an increase in the number of new provings.