How Does Homeopathy Work? A Scoping Review of Proposed Modes of Action

Background: There is currently no generally accepted theory able to explain the observed clinical efficacy of homeopathy. The aim of this scoping review was 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. Methods: Medline, Embase, Scopus, Web of Science, PhilPapers, several online library catalogues, and personal libraries were searched for original studies up to 12th July 2024. Screening and data extraction were performed independently by two reviewers. 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 fourteen overarching theoretical frameworks. Results: A total of 2,118 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: 22% of included publications described theoretical approaches that tried to cover both of the main aspects of homeopathy, while 46% only covered Potentisation, 20% covered only the Principle of Similars, and 11% were non-specific. Conclusion: This scoping review offers an overview of theories and models on the mode of action of homeopathy. These could be classified into 14 largely non-overlapping frameworks. Unexpectedly, the placebo did not emerge as such a framework. In the next step, we aim to perform an in-depth assessment of these theories to tease out the most promising areas for future research.