Adverse effects in homeopathy. A systematic review and meta-analysis of observational studies

Almost all health care interventions have the potential to be associated with risk to patient safety. Different terminologies are used to define treatment induced risk to patient safety and a common definition is the term adverse effect. Beyond the concept of adverse effect and specific to homeopathy is the concept of homeopathic aggravation. Homeopathic aggravation describes a transient worsening of the patients' symptoms, which is not understood as an adverse effect. In order to ensure patient safety within a homeopathic treatment setting, it is important to identify adverse effects, as well as homeopathic aggravations, even though it may be challenging to distinguish between these two concepts. To date there is an obvious lack of systematic information on how adverse effects and homeopathic aggravations are reported in studies. This systematic review and meta-analysis focuses on observational studies, as a substantial amount of the research base for homeopathy are observational.

Eight electronic databases, central webpages and journals were searched for eligible studies. The searches were limited from the year 1995 to January 2020. The filters used were observational studies, human, English and German language. Adverse effects and homeopathic aggravations were identified and graded according to The Common Terminology Criteria for Adverse Effects (CTCAE). Meta-analysis was performed separately for adverse effects and homeopathic aggravations.

A total of 1,169 studies were identified, 41 were included in this review. Eighteen studies were included in a meta-analysis that made an overall comparison between homeopathy and control (conventional medicine and herbs). Eighty-seven percent (n = 35) of the studies reported adverse effects. They were graded as CTCAE 1, 2 or 3 and equally distributed between the intervention and control groups. Homeopathic aggravations were reported in 22,5% (n = 9) of the studies and graded as CTCAE 1 or 2. The frequency of adverse effects for control versus homeopathy was statistically significant (P < 0.0001). Analysis of sub-groups indicated that, compared to homeopathy, the number of adverse effects was significantly higher for conventional medicine (P = 0.0001), as well as other complementary therapies (P = 0.05).

Adverse effects of homeopathic remedies are consistently reported in observational studies, while homeopathic aggravations are less documented. This meta-analysis revealed that the proportion of patients experiencing adverse effects was significantly higher when receiving conventional medicine and herbs, compared to patients receiving homeopathy. Nonetheless, the development and implementation of a standardized reporting system of adverse effects in homeopathic studies is warranted in order to facilitate future risk assessments.