The effectiveness of homeopathy has been in dispute since its inception. One of the earliest double blind studies concerning homeopathy was sponsored by the British government during World War II in which volunteers tested the efficacy of homeopathic remedies against diluted mustard gas burns.
No individual preparation has been unambiguously demonstrated to be different from placebo. The methodological quality of the primary research was generally low, with such problems as weaknesses in study design and reporting, small sample size, and selection bias. Since better quality trials have become available, the evidence for efficacy of homeopathy preparations has diminished; the highest-quality trials indicate that the remedies themselves exert no intrinsic effect. A review conducted in 2010 of all the pertinent studies of "best evidence" produced by the Cochrane Collaboration concluded that "the most reliable evidence – that produced by Cochrane reviews – fails to demonstrate that homeopathic medicines have effects beyond placebo."
- Publication bias and other methodological issues
The fact that individual randomized controlled trials have given positive results is not in contrast with an overall lack of statistical evidence of efficacy. A small proportion of randomized controlled trials inevitably provide false-positive outcomes due to the play of chance: a "statistically significant" positive outcome is commonly adjudicated when the probability of it being due to chance rather than a real effect is no more than 5%—a level at which about 1 in 20 tests can be expected to show a positive result in the absence of any therapeutic effect. Furthermore, trials of low methodological quality (ie ones which have been inappropriately designed, conducted or reported) are prone to give misleading results. In a systematic review of the methodological quality of randomized trials in three branches of alternative medicine, Linde et al. highlighted major weaknesses in the homeopathy sector, including poor randomization.
A related issue is publication bias: researchers are more likely to submit trials that report a positive finding for publication, and journals prefer to publish positive results. Publication bias has been particularly marked in complementary and alternative medicine journals, where few of the published articles (just 5% during the year 2000) tend to report null results. Regarding the way in which homeopathy is represented in the medical literature, a systematic review found signs of bias in the publications of clinical trials (towards negative representation in mainstream medical journals, and vice-versa in complementary and alternative medicine journals), but not in reviews.
- Systematic reviews and meta-analyses of efficacy
Both meta-analyses, which statistically combine the results of several randomized controlled trials, and other systematic reviews of the literature are essential tools to summarize evidence of therapeutic efficacy. Early systematic reviews and meta-analyses of trials evaluating the efficacy of homeopathic remedies in comparison with placebo more often tended to generate positive results, but appeared unconvincing overall. In particular, reports of three large meta-analyses warned readers that firm conclusions could not be reached, largely due to methodological flaws in the primary studies and the difficulty in controlling for publication bias. The positive finding of one of the most prominent of the early meta-analyses, published in The Lancet in 1997 by Linde et al., was later reframed by the same research team, who wrote:
The evidence of bias weakens the findings of our original meta-analysis. Since we completed our literature search in 1995, a considerable number of new homeopathy trials have been published. The fact that a number of the new high-quality trials ... have negative results, and a recent update of our review for the most "original" subtype of homeopathy (classical or individualized homeopathy), seem to confirm the finding that more rigorous trials have less-promising results. It seems, therefore, likely that our meta-analysis at least overestimated the effects of homeopathic treatments.
In 2002, a systematic review of the available systematic reviews confirmed that higher-quality trials tended to have less positive results, and found no convincing evidence that any homeopathic remedy exerts clinical effects different from placebo.
In 2005, The Lancet medical journal published a meta-analysis of 110 placebo-controlled homeopathy trials and 110 matched medical trials based upon the Swiss government's Program for Evaluating Complementary Medicine, or PEK. The study concluded that its findings were compatible with the notion that the clinical effects of homeopathy are nothing more than placebo effects.
A 2006 meta-analysis of six trials evaluating homeopathic treatments to reduce cancer therapy side-effects following radiotherapy and chemotherapy found that there was "insufficient evidence to support clinical efficacy of homeopathic therapy in cancer care".
A 2007 systematic review of homeopathy for children and adolescents found that the evidence for attention-deficit hyperactivity disorder and childhood diarrhea was mixed. No difference from placebo was found for adenoid vegetation, asthma, or upper respiratory tract infection. Evidence was not sufficient to recommend any therapeutic or preventative intervention, and the delay in medical treatment may be harmful to the patient.
In 2012, a systematic review evaluating evidence of homeopathy's possible adverse effects concluded that "homeopathy has the potential to harm patients and consumers in both direct and indirect ways". One of the reviewers, Edzard Ernst, supplemented the article on his blog, writing: "I have said it often and I say it again: if used as an alternative to an effective cure, even the most 'harmless' treatment can become life-threatening."
The Cochrane Library found insufficient clinical evidence to evaluate the efficacy of homeopathic treatments for asthma dementia, or for the use of homeopathy in induction of labor. Other researchers found no evidence that homeopathy is beneficial for osteoarthritis, migraines or delayed-onset muscle soreness.
Health organisations such as the UK's National Health Service, the American Medical Association, and the FASEB have issued statements of their conclusion that there is no convincing scientific evidence to support the use of homeopathic treatments in medicine.
Clinical studies of the medical efficacy of homeopathy have been criticised by some homeopaths as being irrelevant because they do not test "classical homeopathy". There have, however, been a number of clinical trials that have tested individualized homeopathy. A 1998 review found 32 trials that met their inclusion criteria, 19 of which were placebo-controlled and provided enough data for meta-analysis. These 19 studies showed a pooled odds ratio of 1.17 to 2.23 in favor of individualized homeopathy over the placebo, but no difference was seen when the analysis was restricted to the methodologically best trials. The authors concluded "that the results of the available randomized trials suggest that individualized homeopathy has an effect over placebo. The evidence, however, is not convincing because of methodological shortcomings and inconsistencies." Jay Shelton, author of a book on homeopathy, has stated that the claim assumes without evidence that classical, individualized homeopathy works better than nonclassical variations.
In a 2012 article published in the Skeptical Inquirer, Edzard Ernst reviewed the publications of the research group that has published most of the clinical studies of homeopathic treatment from 2005 to 2010. A total of eleven articles, published in both conventional and alternative medical journals, describe three randomized clinical trials (one article), prospective cohort studies without controls (seven articles) and comparative cohort studies with controls (three articles). The diseases include a wide range of conditions from knee surgery, eczema, migraine, insomnia to ‘any condition of elderly patients’. Ernst’s evaluation found numerous flaws in the design, conduct and reporting of the clinical studies. Examples include: little detail of the actual homeopathic treatment administered, misleading presentation of controls (comparison of homeopathic plus conventional treatment and conventional treatment, but presented as homeopathic versus conventional treatment); and published similar data in multiple articles. He concluded that the over- and misinterpreted weak data made the homeopathy appears to have clinical effects which can be attributed to bias or confounding, and that ‘casual reader can be seriously misled’.
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