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Mistletoe and author bias

As our homes are about to become decked with mistletoe for Christmas, Professor Edzard Ernst considers the medicinal use of this interesting plant.

Mistletoe contains lectins (pharmacologically active plant proteins) which have shown promise as a cancer treatment. The amazing story of mistletoe as an anticancer agent goes back about 100 years to Rudolf Steiner, the founder of anthroposophical medicine (see my previous blog).

He felt that, based largely on his personal intuition, mistletoe might cure cancer. Subsequently, it became very well known in Germany where today many cancer patients swear by it.

Systematic research started about 50 years ago and produced surprisingly positive findings. Hundreds of basic research papers suggest anticancer effects. Thus it is reasonable to ask, does it work clinically? The two main therapeutic claims are that mistletoe prolongs life and improves quality of life of cancer patients. Big stuff! But is it true?

Today there are ~40 ‘clinical trials'. I put the term in inverted commas because many are called trials but, in fact, they are merely observational or retrospective studies. Only 20 or so are RCTs.

The evidence is further complicated by the fact that well over a dozen vastly different preparations of mistletoe are on the market (the best-known and most frequently tested is Iscador®, Weleda). Numerous systematic reviews have evaluated the available trial data, but they all struggle with these complexities.

So does it work, or doesn't it? Depending on whom you want to believe, mistletoe is highly effective or it doesn't work beyond a placebo effect. I don't want to bore you with detailed assessments why these contradictions might exist. Yet one finding strikes me as relevant: those evaluations that are authored by authors from anthroposophical institutions are invariably positive. Those that are written by independent authors tend to be much more circumspect and usually fail to arrive at positive conclusions.

Two examples may suffice. The Cochrane review of mistletoe concludes that ‘the evidence from RCTs to support the view that the application of mistletoe extracts has impact on survival or leads to an improved ability to fight cancer or to withstand anticancer treatment is weak' [1].

The latest (of dozens) reviews of anthroposophical proponents of mistletoe therapy, on the other hand, concludes ‘adjuvant treatment of cancer patients with the mistletoe extract Iscador® is associated with a better survival' [2].

What does this teach us? Particularly in areas that are controversial, we need to seriously consider the biases of the authors. I call it the ‘they would say that, wouldn't they' bias.

Professor Edzard Ernst is professor of complementary medicine at the Peninsula Medical School, University of Exeter

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