Homeopathy, from the Greek words homoios (similar) and pathos (suffering), is a controversial system of alternative medicine involving the use of remedies without chemically active ingredients. The theory of homeopathy was developed by the Saxon physician Samuel Hahnemann (1755-1843) and first published in 1796.
Underlying theory The theory of homeopathy holds that every symptom induced by a toxic dose[?] of a substance in a healthy person can be cured by a remedy prepared from that same substance— in Hahnemann's own famous words: similia similibus curentur ("Let like cure likes.") Two example of substances used are Natrium muriatricum (a.k.a. table salt), which is not terribly toxic and Lachesis muta (the venom of the bushmaster snake), which is.
The homeopathic concept of disease differs from that of conventional medicine: The root cause of disease is believed to be spiritual rather than physical, and a disease is thought to manifest itself first in emotional symptoms (e.g. cravings, aversions) and if left untreated gradually progress to mental, modal and finally physical symptoms. As the disease process is thought to begin long before any physical manifestations appear, it logically follows that bacteria and viruses must be effects, not causes, of disease.
A Materia Medica Pura is a listing of symptoms associated with each of a number of substances, produced empirically by homeopathic proving[?] - i.e., the researcher imbibes a toxic dose of the substance and records all physical, mental, emotional and modal symptoms experienced. A homeopathic repertory is a listing of remedies by symptom compiled therefrom, used to determine the most appropriate remedy for a given case. Kent's Repertory (published 1905) lists about 700 different remedies. Today, nearly 3000 different remedies are used in homeopathy, of which approximately 150 are considered common.
The preparation of homeopathic remedies, known as dynamization or potentiziation, consists of successive dilutions followed by shaking in 10 hard strikes against an elastic body at each dilution stage. The vigorous agitation following eacht dilution is thought to transfer some of the spiritual essence of the substance to the water. The dilution factor at each stage is traditonally 1:10 (D or X potencies) or 1:100 (C potencies), though recently LM potencies (dilution factor 1:50,000 at each stage) have been used by some practitioners.
The choice of potency prescribed depends on how deep-seated a disease is diagnosed to be, 12 being a typical starting point for acute conditions compared to 30 for chronic conditions. The dilution factor is considered much less important than the number of successive dilutions - D potencies are generally preferred in Europe, while C potencies prevail in the United States and India.
See also: List of common homeopathic remedies
Homeopathy defines the potency of its remedies according to how diluted they are; the more diluted, the stronger it considers them. The process of dilution is called potentization. The potency is defined in terms of a number, where the higher the number, the higher the dilution. 30X, for example, is more diluted (and thus, according to homeopathy, more potent) than 10X. This is in contrast to conventional medicine and biochemistry, which hold that the more of an active ingredient is present in a drug, the more effect (whether positive or negative) it will have.
Some supporters of homeopathy believe that while lower dilutions may have more of a physiological effect, higher dilutions may have a greater effect on the mental or emotional plane. Even critics would agree that a higher dilution factor probably has its marketing advantages.
Much of the controversy surrounding homeopathy concerns the mechanism that would lie behind the alleged effectiveness of highly diluted substances. Critics argue that homeopathic substances are so diluted as to contain nothing of any value: indeed, that no molecules of the "active" substance remain in the most "potent" dilutions. Defenders of homeopathy, however, argue that the mechanism is irrelevant, because it works; they cite the example of aspirin, which was used for years without anyone knowing how it worked. Critics return that there is a fundamental difference between not understanding the mechanism of a proven medicine, and not understanding any conceivable mechanism for an unproven one. While studies remain controversial, they say, attempting to understand the underlying theory remains important in determining whether homeopathy really has benefits.
At its core, homeopathy is a method of treating diseases and medical conditions invented by the German physician Samuel Hahnemann (1755-1843) in the late 18th and early 19th centuries and significantly refined as well as poularized by the American James Tyler Kent, M.D. It is based on the theory that each naturally occurring element, plant, and mineral compound will, when ingested or applied, result in certain symptoms. Hahnemann believed that, by diluting these substances in a standardized manner, one could reach the true essence of that substance. Hahnemann described this process of dilution as "potentizing" (German: "potenziert") the substance. These dilute amounts could then be used to treat the very symptoms they were known to produce.
Hahnemann and his students approached their treatments in a holistic way, meaning that the whole of the body and spirit is dealt with, not just the localised disease. Hahnemann himself spent extended periods of time with his patients, asking them questions that dealt not only with their particular symptoms or illness, but also with the details of their daily lives. It is also suggested that the gentle approach of homeopathy was a reaction to the violent forms of medicine of the day, which included techniques such as bleeding as a matter of course.
According to homeopaths, conventional (or allopathic) medicine views symptoms as signs of illness (though some modern scientists would see this as an overly simplistic view). Modern treatments are intended to fight disease by targeting the pathogen causing the symptoms. According to homeopathy, however, symptoms are actually the body's way of fighting "dis-ease" (verb not noun.) Homeopathy teaches that symptoms are to be encouraged, by prescribing a "remedy" in minuscule doses that in large doses would produce the same symptoms seen in the patient. These remedies are intended to stimulate the immune system, helping to cure the illness.
Homeopathy has attracted practitioners for more than a century and a half, many of whom have put forth claims of evidence for its efficacy. Homeopathy is rejected as pseudoscience (functioning to some extent through the placebo effect) by the majority of the scientific and medical establishment in the United States and Western Europe. Nevertheless, there is a large market for homeopathic treatments in parts of Europe and in some other nations like India; in Germany, homeopathy and other forms of alternative medicine are covered by health insurance (approval of such remedies does not depend on proven efficacy).
In the United States, homeopathic remedies are subject to regulation by the Food and Drug Administration. Although regulated, the FDA treats homeopathic drugs significantly differently from other drugs. Homeopathic drugs are not required to be approved by the FDA prior to sale, not required to prove either safety or effectiveness prior to being sold, not required to label their products with expiration dates, and not required to undergo finished product testing to verify contents and strength. Homeopathic drugs have their own imprints that, unlike conventional drugs, do not have to identify their active ingredients on the grounds that they have little or no active ingredients. Homeopathic medicines that claim to treat a serious disease can be sold by prescription only, while homeopathic medicines that claim to treat self-limiting conditions may be sold over the counter.
Proponents of conventional medicine charge that patients who rely fully on homeopathic techniques, denying any conventional medicine, are at risk of leaving some easily treatable diseases (such as some early skin cancers) until they become untreatable. It should be mentioned that in many countries (e.g. the United Kingdom), so-called homeopathic medicines are sold over the counter. These medicines rely on the basic theory of treating a symptom with its cause; however, they in no other way resemble the treatments offered by traditional homeopaths. Traditional homeopathy is arguably more recognized and accepted in continental Europe, perhaps because there its practitioners rely on more tradition and treatment with the "potentized" formulae recommended by Hahnemann.
Proponents and opponents of homeopathy disagree over whether scientific randomized controlled trials[?] with the use of placebos have shown success with homeopathic methods. Some clinical trials have produced results supporting homeopathy, but critics contend that these trials are flawed. In 1997, the British medical journal Lancet published a meta-analysis of 89 clinical trials, resulting in an ambiguous conclusion that served as fodder for both supporters and critics of homeopathy.
Many consider homeopathy to be a pseudoscientific remnant from the age of alchemy. The primary results attributed to homeopathy can be explained by the placebo effect. They claim that homeopathic remedies have been scientifically tested (in what is called a "double blind" study to control for placebo effects) many times, and a few of these tests produced slightly positive results. Most scientists easily attribute these to random chance, as the results are only barely measurable, not reliably reproducible, and overwhelmed by the quantity of failed tests. Moreover, the basic way in which tests are carried out means that a small proportion of tests will give false positives. Generally this is statistically protected against, but where lots of tests are done, one or two will appear positive by random chance.
Another criticism of homeopathy is that it is not logically consistent. This theory assumes that water somehow "remembers" the chemical properties of molecules that it once came in contact with. In this practice one dilutes the original solution to the point where one removes all molecules, yet is is claimed that the water retains some chemical properties of the molecule. If this were so, then where did the pure water used in this process come from? The water that homeopaths themselves use once was in contact with other chemicals, including chemical wastes, urine, radioactive metals and various poisons. According the homeopathic theory, all water in the world should "remember" its contact with millions of chemical substances. Yet in practice we find that the homeopathic water remembers absolutely nothing at all, except for the properties of the chemicals that the homeopath claims will be useful.
Although the claims of homeopathy have not been justified by scientific testing, many people widely accept homeopathy due to magical thinking. As Dr. Phillips Stevens points out "Many of today's complementary or alternative systems of healing involve magical beliefs, manifesting ways of thinking based in principles of cosmology and causality that are timeless and absolutely universal. So similar are some of these principles among all human populations that some cognitive scientists have suggested that they are innate to the human species, and this suggestion is being strengthened by current scientific research....Some of the principles of magical beliefs described above are evident in currently popular belief systems. A clear example is homeopathy...The fundamental principle of its founder, Samuel Hahnemann (1755-1843), similia similibus curentur ('let likes cure likes'), is an explicit expression of a magical principle."
Diluting substances as much as homeopathy does would not only vastly decrease any effects the substance in question has, but in fact completely destroy the healing agent. Robert L. Park, Professor of Physics and director of the Washington office of the American Physical Society, writes in his book "Voodoo Science: The Road from Foolishness to Fraud":
[Samuel] Hahnemann [the 18/19th century "inventor" of homeopathy] used a process of sequential dilution and to prepare his medications. He would dilute an extract of some "natural" herb or mineral, one part medicine to ten parts water, or 1:10, shake the solution, and then dilute it another factor of ten, resulting in a total dilution of 1:100. Repeating that a third time gives 1:1000, etc. Each sequential dilution would add another zero. He would repeat the procedure many times. Extreme dilutions are easily achieved by this method. The dilution limit is reached when a single molecule of the medicine remains. Beyond that point, there is nothing left to dilute. In over-the-counter homeopathic remedies, for example, a dilution of 30X is fairly standard. The notation 30X means the substance was diluted one part in ten and shaken, and then this was repeated sequentially thirty times. The final dilution would be one part medicine to 1,000,000,000,000,000,000,000,000,000,000 parts of water. That would be far beyond the dilution limit. To be precise, at a dilution of 30X you would have to drink 7,874 gallons of the solution to expect to get just one molecule of the medicine. Compared to many homeopathic preparations, even 30X is concentrated. Oscillococcinum, the standard homeopathic remedy for flu, is derived from duck liver, but its widespread use in homeopathy poses little threat to the duck population -- the standard dilution is an astounding 200C. The C means the extract is diluted one part per hundred and shaken, repeated sequentially two hundred times. That would result in a dilution of one molecule of the extract to every 10400 molecules of water -- that is, 1 followed by 400 zeroes. But there are only about 1080 (1 followed by 80 zeroes) atoms in the entire universe. A dilution of 200C would go far, far beyond the dilution limit of the entire visible universe! 
Park points out that Hahnemann was likely unaware of exceeding the dilution limit because he did not know about Avogadro's number, a physical constant which makes it possible to calculate the number of molecules in a given mass of a substance. Park explains the early success of homeopathy by comparing it with the use of actually harmful remedies at the same time: "Physicians still treated patients with bleeding, purging, and frequent doses of mercury and other toxic substances. If Hahnehmann's infinitely diluted nostrums did no good, at least they did no harm, allowing the patient's natural defenses to correct the problem."
Park further explains how modern homeopathologists agree that there is no actual molecule of medication in their medicine, but that the liquid "remembers" the substance after the process of dilution. How this substance memory is attained has never been explained. Critics also point out that water spontaneously dissociates into acid and alkali (which is why it has a pH of 7). The quantity of acid in a homeopathic remedy, although tiny, is generally higher than the quantity of active agent.
The NCAHF Position Paper on Homeopathy was adopted February 1994 by The National Council Against Health Fraud, a private non-profit organization. Permission to reprint is granted with proper citation. Below are excerpts from the paper, followed by a link to the complete article.
NCAHF Position Paper on Homeopathy (http://www.ncahf.org/pp/homeop)
Proponents of homeopathic treatment point to the fact that the vast majority of people who do seek out professional homeopathic treatment are satisfied with the results. It is further argued that any treatment which makes a patient better is a valid, regardless of whether or not it conflicts with the currently accepted model of the molecular composition of matter. Discussion of whether or not the placebo effect is involved are likewise considered academic, as a cured patient is a cured patient either way. The most potent examples of cases in which the placebo effect could not be used to explain the results are those involving infants and young children, or animals.
Recent research indicates that in certain situations the further diluted a substance, the more its molecules tend to clump together . Some would like to see this as evidence supporting homeopathic therapies. However this data doesn't explain why the substances need to be diluted, just that they might remain active after this preparation (not in the non-concentrations of homeopathic medicine, though). Further, this phenomenon has no connection to homeopathy because in these cases there is no attempt to dilute the molecule away to nothing. Homeopathy attempts to dilute molecules away until none are left, while these experiments always maintained measurable amounts of molecules in their solutions. These experiments merely examined the difference in properties that molecules have when clumped together in aggregates and polymers, rather than as smaller polymemers and monomers.
Dana Ullman, in his 1995 book, "The Consumer's Guide to Homeopathy", devotes an entire chapter to "Scientific Evidence for Homeopathic Medicine". For example, he cites a 1991 study, in which he writes:
three professors of medicine from the Netherlands, none of them homeopaths, performed a meta-analysis of twenty-five years of clinical studies using homeopathic medicines and published their results in the journal British Medical Journal. This meta-analysis covered 107 controlled trials, of which 81 showed that homeopathic medicines were effective, 24 showed they were ineffective, and 2 were inconclusive. The professors concluded, "The amount of positive results came as a surprise to us." 
Critics of homeopathy held that these so-called "effective" results were tiny, unrepeatable, and poorly controlled.
Some homeopathic practitioners may ascribe the lack of definitive support from controlled trials to the the absence of an emotional doctor-patient bond that is necessary in order for treatment to be successful (an argument, opponents claim, that is common to religion and pseudosciences and contradicts the scientific method). Other homeopathic practitioners, however, believe that research does justify the effectiveness of homeopathy, and Ullman has argued that clinical research need not be invalidated by the need for a tailored remedy for a given individual. For example, he cites an article published in the December 10, 1994, issue of Lancet ("Is Evidence for Homeopathy Reproducible?"), which documents a clinical trial concerning the use of homeopathic remedies to treat asthma. He also cited several other trials, such as one involving children with diarrhea, documented in the May, 1994 issue of Pediatrics ("Treatment of Acute Childhood Diarrhea with Homeopathic Medicine: A Randomized Clinical Trial in Nicaragua"). This approach, with its willingness to make falsifiable predictions, is more characteristic of protoscience than pseudoscience.
Ullman, in fact, argues that studies have confirmed that homeopathic remedies are effective even without personalized treatment in a practitioner-patient relationship. He cites two studies, including one published in the March, 1989 issue of British Journal of Clinical Pharmacology ("A Controlled Evaluation of a Homeopathic Preparation for the Treatment of Influenza-like Syndrome"), to bolster this position. So-called "combination remedies", in which several homeopathic preparations are combined, are often sold over-the-counter in the United States, and traditional homeopathic theory tends to frown on this approach, but Ullman cites trials that suggest otherwise.
Ullman argues, in fact, "to ignore the body of experimental data that presently exist on homeopathic medicines and to deny the body of clinical experience of homeopaths and homeopathic patients, one would have to be virtually blind. One can only assume that this blindness is a temporary affliction, one that will soon be cured." Scientists looking at the same data deny that these were properly controlled experiments.
"In 1988, a French scientist working at that country's prestigious INSERM institute claimed to have found that high dilutions of substances in water left a 'memory,' providing a rationale for homeopathy's Law of Infinitesimals. His findings were published in a highly regarded science journal, but with the caveat that the findings were unbelievable, and that the work was financed by a large homeopathic drug manufacturer (Nature, 1988). Subsequent investigations, including those by James Randi, disclosed that the research had been inappropriately carried out. T he scandal resulted in the suspension of the scientist." (Source: National Council Against Health Fraud position paper on Homeopathy; Permission to reprint is granted with proper citation.)
Despite these claims, debate continues on the results of further trials, as it likely will as long as homeopathy is a flourishing business.