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Mistletoe Can be Highly Toxic to Humans

There's been a mystique around mistletoe since ancient times, probably on account of the curious way it grows. The plant is a “hemiparasite,” meaning that it can either grow in soil, or, more commonly, it can spring from the branch of a tree. How did "kissing under the mistletoe" become a thing? People probably stood under the branch in awe, admiring the pretty flowers, giving others an opportunity to take a little liberty with their smooches.

The original mistletoe, Viscum album, (different from the ornamental North American version) got its name from the Anglo-Saxon word “mistel” for “dung” and “tan” for “twig.” Dung-on-a twig really is an excellent description of the plant’s origin. Mistletoe would often appear on a branch where birds left their droppings which contained mistletoe berry seeds that had passed through their digestive tract. Interestingly, birds are not bothered by the seeds which are highly toxic to humans. The main culprits are “viscotoxins,” small proteins than can destroy cells.

Any substance that has such effects on human health arouses scientific curiosity. Pharmaceutical history is peppered with attempts to use small doses of poisons to wipe out a disease without wiping out the patient. Arsenic, mercury, strychnine and belladonna are obvious examples. So it should come as no surprise that various mistletoe preparations also appeared in drug compendia. Until the 1920s, these remedies were dismissed by the scientific community as mere placebos. But then researchers discovered that mistletoe also harbours some complex compounds called lectins that can bind to cells and induce biochemical changes. Attention now focused on the possibility that these substances at the right concentration might selectively destroy cancer cells. Early on there was encouragement from laboratory studies and animal trials that showed a slowing of the growth of certain tumours in response to mistletoe extracts. This was enough for the producers of herbal products to get their bandwagons rolling and load them up with mistletoe extracts with intriguing names like Iscador, Eurixor or Helixor. Unfortunately, human trials have not born out the early optimism and there is no evidence from properly controlled trials that such products have a beneficial effect on cancer. There is, however, plenty of evidence that they don’t. It seems that mistletoe’s magic is limited to enticing people to express their affections for each other. And that’s nothing to scoff at.


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