Animal experiments form the largest part of homeopathic basic research: about half of all published basic research investigations used animal models. Due to the large number of publications and studies (currently more than 1000), this research field has not been completely reviewed. There are general systematic reviews covering more recent years, as well as some topic-specific meta-analyses in the areas of experimental toxicology, behavior, and metamorphosis. Due to the lack of a comprehensive review of the entire field of animal experiments, it is not possible to give an overall assessment of the state of research in this field. The three available meta-analyses on experimental toxicology, behavior, and development point to specific effects of homeopathic preparations over placebo.
Van Wijk et al. published a bibliographic overview of the research literature involving animal experimentation. They concluded that animal experiments form the biggest part of homeopathic basic research: about half of all published basic research investigations used animal models. The studies mostly assessed efficacy and effectiveness of potentized preparations as well as the applicability of the Simile Principle. Most studies used rats or mice, fewer studies were performed with guinea pigs, frogs, rabbits, pigs, chicken, or other animals.
Recent systematic reviews on animal experiments in general were published by Bonamin et al., covering the years 1999–2014 (Review 1, Review 2). The experimental models were applied to the following areas: toxicology, behavior, inflammation, carcinogenesis, metamorphosis, and infection. In total, 84 publications were identified, out of which 80 (95%) observed significant results of homeopathic preparations against controls. Randomization was declared in 65 publications (77%) and blinding in 44 (52%) out of 84 publications.
In an older systematic review and meta-analysis of experimental toxicology, 75 publications describing 95 animal experiments were identified. Quality was assessed with a specifically developed score. Only experiments with a score higher than the median were selected for further in-depth analysis. Amongst these experiments of higher quality, three animal models define most of the remaining studies. These investigations studied the effects of homeopathic treatments on (i) arsenic concentration in body fluids following arsenic poisoning, (ii) mortality after HgCl2 poisoning, and (iii) hepatitis after CCl4 poisoning. Meta-analysis could be performed for experiments involving rats studying arsenic concentration in blood, urine, and feces after arsenic poisoning and after treatment with potentized arsenic 7c, yielding statistically significant increased concentration of arsenic in urine and feces, and decreased arsenic concentration in blood. Another meta-analysis could be calculated for the mortality of mice after HgCl2 poisoning, yielding a decrease in mortality by 40% and 7% after application of mercury chloride 15c and 9c, respectively.
A systematic review of rodent behavioral and psychopathological models revealed 18 publications between 1960 and 2009, reporting on studies with models of anxiety, catalepsy, convulsion, and others, involving mice, rats, and guinea pigs. Only few results were confirmed by multiple laboratories, and concern potentized Ignatia, Gelsemium, and Chamomilla. Bellavite et al. furthermore report on a meta-analysis of studies of potentized Gelsemium sempervirens in different anxiety related models performed in their own laboratory. Pooled analysis demonstrated highly significant anxiolytic effects of Gelsemium 5c, 7c, 9c and 30C on different outcome parameters, without any sedative action or adverse effects on locomotion. Comparison with conventional anxiolytic drugs revealed a broader action of Gelsemium.
In 1990, Endler et al. introduced an animal development model into homeopathic basic research. In this model, tadpoles were treated with Thyroxine 30x and speed of metamorphosis from the 2-legged to the 4-legged stage was investigated. A meta-analysis of 24 independent experiments performed in the years 1990–2013 involving seven researchers and nearly 2000 animals, yielded highly significant evidence for specific effects of Thyroxine 30x over placebo by slowing down the metamorphosis with a medium to large effect size d.
Summarizing, it is not possible to give an overall statement of the state of research in this area due to the lack of a comprehensive and thorough review of the entire field of animal experiments in homeopathic basic research. The available meta-analyses on specific research areas such as experimental toxicology, behavior, and development are in favor of specific effects of homeopathic preparations over placebo.