Conducting systematic reviews of complementary and alternative medicine: common pitfalls.

Systematic reviews (SRs) are considered the best tools for summarizing the evidence for or against the effectiveness of health care interventions. The principles and methods of SRs apply equally to both, mainstream and complementary/alternative medicine (CAM). Certain challenges are, however, more commonly encountered in CAM or even specific to it; this article is aimed at raising awareness of these among systematic reviewers. When searching for literature, specific issues relating to specialist databases, indexing, access, foreign language studies, and certain forms of publication bias need to be considered. Researchers also need to be aware of the difficulties of comparing CAM studies and address the variability between studies. CAM modalities are highly diversified and great variations exist in the standardization of herbal products and other dietary supplements. Individualization of treatment as well as different classifications of disease and different diagnostic methods need to be addressed. Expectation bias is high in CAM, and finding appropriate controls and blinding are often challenging. It is important that these issues are taken into account early on in the planning stages of an SR so that proper consideration can be given to the search strategies, inclusion/exclusion criteria and methods of analysis with the overall aim of reducing bias.