Mapping the Theories and Models on the Mode of Action of Homeopathy: A Scoping Review

Background: There is currently no generally accepted theory able to explain the observed clinical efficacy of homeopathy. The aim of this scoping review is to identify all theoretical approaches that have been used to explain homeopathy, with the objective of establishing a basis for identifying promising hypotheses and theories for future elaboration. Methods: Medline, Embase, Scopus, Web of Science, PhilPapers, several online library catalogs, and personal libraries were searched for original studies up to July 12, 2024. Screening and data extraction were performed independently by two reviewers. Publications were included if they developed or advanced theories or models related to homeopathy. The aspect of homeopathy addressed by each study was extracted: the Principle of Similars and/or Potentisation. The theories identified in this review could be grouped into 14 overarching theoretical frameworks. Results: In total, 2118 records were screened, 500 full texts were assessed for eligibility, and 216 studies were included in this review. Starting in 1832 until the late 20th century, only sporadic contributions were found. From the 1990s, a marked increase in scholarly output was recorded. Most first authors were established in Germany, the United States, the United Kingdom, or India. The frameworks with the highest number of contributions were humanities, complex systems, water structures, and weak quantum theory. Overall, 22% of included publications described theoretical approaches that tried to cover both main aspects of homeopathy, 46% only Potentisation, 20% only the Principle of Similars (and 11% were unspecific). The theories within the humanities framework focused more on the Principle of Similars, complex systems theories on both principles, and the theories within the water structures, nanostructures, and mathematical models frameworks focused mostly on the Potentisation aspect. Conclusions: This scoping review offers an overview of theories and models on the mode of action of homeopathy. These could be classified into 14 largely nonoverlapping frameworks. Unexpectedly, the placebo did not emerge as such a framework. In the next step, these theories would be assessed in terms of their quality, plausibility, compatibility with modern science, and experimental falsifiability.