Can you trust systematic reviews of complementary and alternative therapies?

Systematic reviews have considerable impact on the discussion of complementary therapies in the scientific community, in the media and on healthcare decision making, because they are considered to be the most reliable tool to summarize and assess the evidence available on a defined question. This article aims to discuss some of the most relevant problems of systematic reviews, both in general and in relation to complementary and alternative medicine (CAM). Review questions in CAM, for example, tend to be broad which often leave the results open to subjective interpretation. To search the literature comprehensively, it is sometimes necessary to go beyond conventional electronic databases, but the quality of the material identified in other sources is often doubtful. Small changes in selection criteria can strongly influence the inclusion of studies. Publication bias, insufficient reporting in primary studies, and problems with quality assessment are relevant to both conventional medicine and CAM. While systematic reviews are, for the time being, without alternative, they have to be read and interpreted with caution. In many cases, particularly in the area of CAM, they will leave room for controversial discussions.