Experimental studies

Up to the end of 2010 a total of 156 randomised controlled trials (RCTs) in homeopathy have been published in good quality scientific journals: positive effects have been reported in 64 (41% of the total) and negative findings have been reported in 11 (7%), while 81 (52%) have not been conclusively positive or negative. That research is spread rather thinly over 75 different medical conditions. In 36 of those conditions, there has been replicated research (i.e. there have been two or more trials); for each of the other 39 conditions, there are singleton RCTs. Differing study designs and the small size of many trials means that there are few conditions where there has been an opportunity to achieve consistent results. An overview of all RCTs can be found here.

Systematic reviews and meta-analyses focused on specific medical conditions

Systematic reviews are focused on a research question that tries to identify, appraise, select and synthesise all high quality research evidence relevant to that question. Systematic reviews often, but not always, use statistical techniques (meta-analysis) to combine results of the eligible studies, or at least use scoring of the levels of evidence depending on the methodology used. To date 26 condition-specific reviews have been published; ten of these 26 reviews have been positive for homeopathy (the other 16 are largely inconclusive):

- allergies and upper respiratory tract infections
Bornhöft G, Wolf U, von Ammon K, Righetti M, Maxion-Bergemann S, Baumgartner S, Thurneysen AE, Matthiessen PF (2006). Effectiveness, safety and cost-effectiveness of homeopathy in general practice – summarized health technology assessment. Forschende Komplementärmedizin, 13 Suppl 2: 19–29 [PubMed]
Bellavite P, Ortolani R, Pontarollo F, Piasere V, Benato G, Conforti A (2006) Immunology and homeopathy. 4. Clinical studies – Part 1. Evidence-based Complementary and Alternative Medicine; eCAM, 3: 293-301 [PubMed]

- childhood diarrhoea
Jacobs J, Jonas WB, Jimenez-Perez M, Crothers D (2003). Homeopathy for childhood diarrhea: combined results and metaanalysis from three randomized, controlled clinical trials. Pediatric Infectious Disease Journal, 22:229–234 [PubMed]

- influenza
Vickers A, Smith C (2006). Homoeopathic Oscillococcinum for preventing and treating influenza and influenza-like syndromes (Cochrane Review). In: The Cochrane Library. Chichester, UK: John Wiley & Sons, Ltd. CD001957 [PubMed]

- post-operative ileus
Barnes J, Resch K-L, Ernst E (1997). Homeopathy for postoperative ileus? A meta-analysis. Journal of Clinical Gastroenterology, 25:628–633 [PubMed]

- rheumatic diseases
Jonas WB, Linde K, Ramirez G (2000). Homeopathy and rheumatic disease. Rheumatic Disease Clinics of North America, 26: 117–123 [PubMed]

- seasonal allergic rhinitis
Wiesenauer M, Lüdtke R (1996). A meta-analysis of the homeopathic treatment of pollinosis with Galphimia glauca. Forschende Komplementärmedizin und Klassische Naturheilkunde, 3:230–236 [PubMed]
Taylor MA, Reilly D, Llewellyn-Jones RH, McSharry C, Aitchison TC (2000). Randomised controlled trials of homoeopathy versus placebo in perennial allergic rhinitis with overview of four trial series. British Medical Journal, 321:471–476 [PubMed]
Bellavite P, Ortolani R, Pontarollo F, Piasere V, Benato G, Conforti A (2006). Immunology and homeopathy. 4. Clinical studies – Part 2. Evidence-based Complementary and Alternative Medicine: eCAM, 3:397–409 [PubMed]

- vertigo
Schneider B, Klein P, Weiser M (2005). Treatment of vertigo with a homeopathic complex remedy compared with usual treatments: a meta-analysis of clinical trials. Arzneimittelforschung, 55:23–29 [PubMed]

Nine reviews were non-conclusive:
- anxiety
Pilkington K, Kirkwood G, Rampes H, Fisher P, Richardson J (2006). Homeopathy for anxiety and anxiety disorders: A systematic review of the research. Homeopathy, 95: 151–162 [PubMed]

- cancer side-effects
Kassab S, Cummings M, Berkovitz S, van Haselen R, Fisher P (2009). Homeopathic medicines for adverse effects of cancer treatments (Cochrane Review). In: The Cochrane Library. Chichester, UK: John Wiley & Sons, Ltd. CD004845 [PubMed]

- chronic asthma
McCarney RW, Linde K, Lasserson TJ (2004). Homeopathy for chronic asthma (Cochrane Review). In: The Cochrane Library. Chichester, UK: John Wiley & Sons, Ltd. CD000353 [PubMed]

- dementia
McCarney R, Warner J, Fisher P, van Haselen R (2004). Homeopathy for dementia (Cochrane Review). In: The Cochrane Library. Chichester, UK: John Wiley & Sons, Ltd. CD003803 [PubMed]

- depression
Pilkington K, Kirkwood G, Rampes H, Fisher P, Richardson J (2005). Homeopathy for depression: a systematic review of the research evidence. Homeopathy, 94:153–163 [PubMed]

- headache and migraine treatment
Owen JM, Green BN (2004). Homeopathic treatment of headaches: A systematic review of the literature. Journal of Chiropractic Medicine, 3:45–52 [Abstract]

- HIV/AIDS
Ullman D (2003). Controlled clinical trials evaluating the homeopathic treatment of people with human immunodeficiency virus or acquired immune deficiency syndrome. Journal of Alternative and Complementary Medicine, 9: 133–141 [PubMed]

- induction of labour
Smith CA (2003). Homoeopathy for induction of labour (Cochrane Review). In: The Cochrane Library. Chichester, UK: John Wiley & Sons, Ltd. CD003399 [PubMed]

- osteoarthritis
Long L, Ernst E (2001). Homeopathic remedies for the treatment of osteoarthritis – A systematic review. British Homeopathic Journal, 90:37–43 [PubMed]

Little or no evidence for homeopathy has been the conclusion in:
- ailments of childhood and adolescence
Altunç U, Pittler MH, Ernst E (2007). Homeopathy for childhood and adolescence ailments: systematic review of randomized clinical trials. Mayo Clinic Proceedings, 82: 69–75 [PubMed]

- attention deficit hyperactivity disorder
Coulter MK, Dean ME (2007). Homeopathy for attention deficit/hyperactivity disorder or hyperkinetic disorder (Cochrane Review). In: The Cochrane Library. Chichester, UK: John Wiley & Sons, Ltd. CD005648 [PubMed]

- cancer treatment
Milazzo S, Russell N, Ernst E (2006). Efficacy of homeopathic therapy in cancer treatment. European Journal of Cancer, 42: 282–289 [PubMed]

- delayed-onset muscle soreness
Ernst E, Barnes J (1998). Are homoeopathic remedies effective for delayed-onset muscle soreness? - A systematic review of Placebo-controlled trials. Perfusion (Nürnberg), 11: 4–8

- headache and migraine prevention
Ernst E (1999). Homeopathic prophylaxis of headaches and migraine? A systematic review. Journal of Pain and Symptom Management, 18:353–357 [PubMed]

- influenza prevention
Vickers A, Smith C (2006). Homoeopathic Oscillococcinum for preventing and treating influenza and influenza-like syndromes (Cochrane review). In: The Cochrane Library. Chichester, UK: John Wiley & Sons, Ltd. CD001957 [PubMed]

Some non-replicated RCTs positive for homeopathy

For other medical conditions, the published evidence is fragmentary: there are some non-replicated RCTs showing positive effects for homeopathy, e.g. in acute otitis media, attention deficit hyperactivity disorder (ADHD), fybromyalgia, stomatitis, chronic fatigue syndrome, sepsis, and post-partum bleeding.
Jacobs J, Springer DA, Crothers D (2001). Homeopathic treatment of acute otitis media in children: a preliminary randomized placebo-controlled trial. Pediatric Infectious Disease Journal, 20:177–183 [PubMed]
Frei H, Everts R, von Ammon K, Kaufmann F, Walther D, Hsu-Schmitz SF, Collenberg M, Fuhrer K, Hassink R, Steinlin M, Thurneysen A (2005). Homeopathic treatment of children with attention deficit hyperactivity disorder: a randomized, double blind, placebo controlled crossover trial. European Journal of Pediatrics, 164:758–767 [PubMed]
Bell IR, Lewis DA 2nd, Brooks AJ, Schwartz GE, Lewis SE, Walsh BT, Baldwin CM (2004). Improved clinical status in fibromyalgia patients treated with individualized homeopathic remedies versus placebo. Rheumatology (Oxford), 43:577-582 [PubMed]
Oberbaum M, Yaniv I, Ben-Gal Y, Stein J, Ben-Zvi N, Freedman LS, Branski D (2001). A randomized, controlled clinical trial of the homeopathic medication Traumeel S in the treatment of chemotherapy-induced stomatitis in children undergoing stem cell transplantation. Cancer, 92:684–690 [PubMed]
Weatherley-Jones E, Nicholl JP, Thomas KJ, Parry GJ, McKendrick MW, Green ST, Stanley PJ, Lynch SP (2004). A randomized, controlled, triple-blind trial of the efficacy of homeopathic treatment for chronic fatigue syndrome. Journal of Psychosomatic Research, 56:189–197 [PubMed]
Frass M, Linkesch M, Banyai S, Resch G, Dielacher C, Lobl T, Endler C, Haidvogl M, Muchitsch I, Schuster E (2005). Adjunctive homeopathic treatment in patients with severe sepsis: a randomized, double-blind, placebo-controlled trial in an intensive care unit. Homeopathy, 94:75–80 [PubMed]
Oberbaum M, Galoyan N, Lerner-Geva L, Singer SR, Grisaru S, Shashar D, Samueloff A (2005). The effect of the homeopathic remedies Arnica and Bellis perennis on mild postpartum bleeding – a randomized, double-blind, placebo-controlled study –preliminary results. Complementary Therapies in Medicine, 13:87–90 [PubMed]

On the other hand, there are some conditions for which trial evidence has been non-conclusive or negative.

Comprehensive systematic reviews and meta-analyses of the whole homeopathic literature

Five comprehensive systematic reviews of homeopathy research covering over 75 different medical conditions have been published. The Cochrane Handbook for Systematic Reviews recommends "Meta-analysis should only be considered when a group of trials is sufficiently homogeneous in terms of participants, interventions and outcomes to provide a meaningful summary". This means that systematic reviews are far from appropriate when trials are extremely heterogeneous not only in results but also in the interventions and health conditions under study and when a therapeutic system works in some but not all indications. The small number of original research papers, the differing criteria reviewers have used for data extraction, the disparate styles of homeopathy used, and the fact that a diverse range of medical conditions has been examined collectively, all severely limit the value of formal comprehensive systematic reviews in homeopathy. Still, four out of five major comprehensive systematic reviews of RCTs in homeopathy have concluded that homeopathy has an effect greater than placebo.

The first comprehensive systematic review of homeopathy research was published in the British Medical Journal in 1991. Of the 105 trials with interpretable results, 81 indicated positive results, which included RCTs that received high quality ratings for randomization, blinding, sample size, and other methodological criteria. The authors, three orthodox epidemiologists, came to the following conclusion: "The amount of positive evidence even among the best studies came as a surprise to us. Based on this evidence we would readily accept that homeopathy can be efficacious, if only the mechanism of action were more plausible". And "The evidence presented in his review would probably be sufficient for establishing homeopathy as a regular treatment for certain indications".
Kleijnen J, Knipschild P, ter Riet G (1991). Clinical trials of homeopathy, British Medical Journal, 302: 316-323[PubMed]

In 1996, a comprehensive review by the European Commission Homeopathic Medicine Research Group, later published in a scientific journal (Cucherat), identified 184 controlled clinical trials. They selected the highest quality randomized placebo controlled trials, which included a total of 2,001 patients. Boissel used a rather unusual technique of combining p-values of the selected trials to answer the question: 'Does homeopathy have an effect?' and concluded that homeopathy was more effective than placebo though "the strength of this evidence is low because of the low methodological quality of the trials".
 A full report and update of this review, with at least two new trials added to the meta-analysis and a total of 2,579 patients, found “some evidence that homeopathic treatments are more effective than placebo” (P < 0.001); it concluded that “the strength of this evidence is low because of the low methodological quality of the trials”.
Boissel JP, Cucherat M, Haugh M, Gauthier E (1996). Critical literature review on the effectiveness of homeopathy: overview of data from homeopathic medicine trials. In: Homeopathic Medicine Research Group, Report of the, Commission of the European Communities, Directorate-General XII – Science, Research and Development, Directorate E – RTD Actions: Life Sciences and Technologies – Medical Research, Brussels 1996; Chap.11: 195-210
Cucherat M, Haugh MC, Gooch M, Boissel JP (2000). Evidence of clinical efficacy of homeopathy – A meta-analysis of clinical trials. European Journal of Clinical Pharmacology, 56:27–33 [PubMed]

A comprehensive systematic review and meta-analysis published in The Lancet in 1997 included 186 placebo controlled studies of homeopathy, from which data for analysis could be extracted from 89. The overall mean odds ratio for these 89 clinical trials was 2.45 (95% confidence interval 2.05–2.93) in favour of homeopathy, which means that the chances of homeopathy giving benefit were 2 to 3 times greater than those of placebo. Even when considering a "worst-case" scenario in which only high-quality studies, of high or medium dilutions, published in MEDLINE-listed journals, and with predefined measures of primary outcome were analysed, the pooled odds ratio was 1.97 and remained statistically significant. The main conclusion was that the results "were not compatible with the hypothesis that the effects of homoeopathy are completely due to placebo". In further analysis, the authors clarified that higher quality trials were less likely to be positive than those of lower quality – which is comparable to the conclusion from similar analyses in conventional medicine –, though the difference from placebo remained statistically significant until analysis was restricted to just five trials. There was insufficient volume of evidence to enable conclusions to be drawn about what homeopathic treatment is effective in which diagnosis.
Linde K, Clausius N, Ramirez G, Melchart D, Eitel F, Hedges LV, Jonas WB (1997) Are the clinical effects of homeopathy placebo effects? A meta-analysis of placebo-controlled trials. Lancet, 350: 834-843 [PubMed]

Linde and Melchart published another review in 1998, based on the same set of studies as the Linde et al. study of 1997. Using more stringent inclusion criteria, they concentrated on 32 trials of homeopathy and concluded in favour of an effect of homeopathy, but stressed the weakness of the evidence and the need for more high-quality trials of homeopathy.
Linde K, Melchart D (1998) Randomized controlled trials of individualized homeopathy: a state-of-the-art review. Journal of Alternative and Complementary Medicine, 4:371-388 [PubMed]

In 2005 an other comprehensive review was published in The Lancet comparing 110 placebo controlled trials of homeopathy and 110 matched trials of conventional medicine. Homeopathy and conventional medicine showed a similar positive treatment effect overall. Twenty-one homeopathy trials and 9 in conventional medicine were judged of higher methodological quality. From these, the results of 14 unspecified "larger trials of higher quality" (8 homeopathy, 6 conventional medicine) were analysed. The mean odds ratio was 0.88 (95% CI, 0.65-1.19) for the 8 homeopathy trials, and 0.58 (95% CI, 0.39-0.85) for the 6 conventional medicine trials. In this study, in contrast with the 1997 review, an odds ratio less than 1.0 indicated an effect greater than placebo. The authors concluded there was "weak evidence for a specific effect of homoeopathic remedies, but strong evidence for specific effects of conventional interventions. This finding is compatible with the notion that the clinical effects of homoeopathy are placebo effects".
The authors' analysis has been criticised to be prone to selection bias, especially when the set of 21 high-quality trials was reduced to 8 trials with large patient numbers. Obviously, the results depend on how the threshold for "large" studies was defined from these 21 studies. The overall results – and the conclusions drawn from them – change depending on which subset of homeopathic trials is analysed. The choice of other meaningful subsets could lead to the opposite conclusion. For example, four of the 21 trials dealt with preventing or treating muscle soreness and an earlier systematic review already showed that homeopathic medicines are probably not helpful in this condition. When restricting the analysis to the remaining 17 trials an overall statistically significant effect can be found. In addition, there is no external criterion why a "large" trial should have N=98 or more patients as defined by the authors (apart from being able to include a statistically non-significant trial with a sample size of exactly 98). For example, if the data set was split at a threshold of N=66, the median sample size of all 110 homeopathic trials, there had been a significant effect in favour of homeopathy. Surprisingly, the authors themselves highlight – but dismiss – the fact that 8 trials of homeopathy in upper respiratory tract infections have strongly positive findings overall. Clearly, the conclusions by this group of scientists under the direction of Professor Matthias Egger, well known for his anti-homeopathy standpoint, are not so definite as they have been reported and discussed.
Shang A, Huwiler-Muntener K, Nartey L, Juni P, Dorig S, Sterne JA, Pewsner D, Egger M (2005). Are the clinical effects of homoeopathy placebo effects? Comparative study of placebo-controlled trials of homoeopathy and allopathy. Lancet, 366:726–32 [PubMed]
Lüdtke R, Rutten AL (2008) .The conclusions on the effectiveness of homeopathy highly depend on the set of analyzed trials. Journal of Clinical Epidemiology, 61:1197-1204 [PubMed]
Rutten AL, Stolper CF (2008) The 2005 meta-analysis of homeopathy: the importance of post-publication data. Homeopathy, 97:169-177 [PubMed]