A total of 163 randomised controlled trials (RCTs) in homeopathy have been published in good quality scientific journals: positive effects have been reported in 41% of the total, 7% have a balance of negative evidence, while 52% have not been conclusively positive or negative. That research is spread rather thinly over 77 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. There are 35 medical conditions in which positive conclusions for homeopathy may be derived from systematic reviews and/or randomised controlled trials; they can be found here.
The above figures have similarities to data obtained from an analysis of 1,016 systematic reviews of RCTs (and therefore of many more than that number of RCTs in total): 44% of the reviews concluded that the interventions studied were likely to be beneficial (positive), 7% concluded that the interventions were likely to be harmful (negative), and 49% reported that the evidence did not support either benefit or harm (non-conclusive).
Difficulties with RCTs
The RCT model of measuring efficacy of a drug poses some challenges for homeopathic research. In homeopathy, treatment is usually tailored to the individual. A homeopathic prescription is based not only on the symptoms of disease in the patient but also on a host of other factors that are particular to that patient, including lifestyle, emotional health, personality, eating habits and medical history. The 'efficacy' of an individualised homeopathic intervention is thus a complex blend of the prescribed medicine together with the other facets of the in-depth consultation and integrated health advice provided by the practitioner; under these circumstances, the specific effect of the medicine itself may be difficult to quantify with precision in RCTs.
An alternative research approach, which the majority of researchers have adopted, is the 'one drug fit all patients' type of RCT. Such trials are capable of quantifying efficacy of the homeopathic 'drug' under investigation, but they may yield results that are of questionable relevance to the practice of homeopathy in the 'real world'.
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 29 condition-specific reviews have been published; eleven of these 29 reviews have been positive for homeopathy, 13 reviews were non-conclusive, 6 reviews found little or no evidence for homeopathy. References to these reviews can be found here.
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. References to these reviews can be found here.