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Has no proved benefit, and can cause harm
Most doctors in the United Kingdom will be surprised to learn from a case reported in this week's BMJ of a use for mistletoe (Viscum album) that has nothing to do with Christmas.1 Some patients with cancer inject themselves with extract of mistletoe in the hope of improving their condition. In continental Europe, at least 30 different mistletoe preparations are available. In Europe, most cancer patients use such extracts, at a total expense of about £30m (€45m; $59m) each year,2 and in Germany the insurance system pays for this treatment.
A Google search (20 November 2006) showed that 145000 websites promote or mention mistletoe as a treatment for cancer. This much publicity may mean that many cancer patients in the UK will try mistletoe in the future or ask their doctor about it. It is therefore timely to discuss the value of mistletoe as an anticancer drug.
A century ago, Rudolf Steiner developed anthroposophy, a school of thought that led to innovations such as the Waldorf schools, biodynamic farming, and anthroposophic medicine. This approach to healthcare is based on intuitive thinking about assumed associations between four postulated dimensions of the human body (physical body, etheric body, astral body, and ego), plants, minerals, and the cosmos.3
Anthroposophic medicine includes drugs, art therapy, rhythmic massages, special exercises, external applications, counselling, and anthroposophic nursing. These treatments are used “partly as adjuncts to and partly as substitutes for conventional medicine.”4 Anthroposophic drugs are based on ancient alchemistic and homeopathic notions, far removed from the concepts of pharmacology. Many of these drugs are produced in unusual ways—some mistletoe preparations are fermented while other anthroposophic drugs are highly diluted according to homeopathic principles.
Steiner's intuition that mistletoe might help treat cancer is based on the fact that, like cancer, mistletoe is a parasitic growth that eventually kills its host. Inspired by Hahnemann's “like cures like” principle, he believed that an extract of mistletoe would cure cancer. Despite the implausibility of this idea, about 1000 in vitro studies have shown that mistletoe or its main constituents (alkaloids, lectins, and viscotoxins) do have anticancer activity.2 5 However, many plants have some sort of anticancer activity.6 Occasionally, this is useful therapeutically—vinblastine and vincristine are derived from the common periwinkle and Taxol comes from the yew tree. In most cases though, toxicity or lack of bioavailability prohibit the use of these compounds.
Proponents of anthroposophic medicine make two claims about mistletoe. Firstly, they claim that regular injections of mistletoe extract improve the natural course of cancer by slowing down or stopping tumour growth. Secondly, they say that such extracts improve the quality of life in patients with cancer.4
Many clinical studies of mistletoe exist, but their findings are inconsistent. Most of them are methodologically weak, and the less rigorous they are the greater the likelihood of a positive result. The conclusions of systematic reviews are therefore contradictory. Anthroposophical doctors, who tend to include unreliable primary studies, arrive at positive conclusions.4 In contrast, independent reviewers tend to focus on the most reliable evidence and regularly find that neither of the above two claims is supported by good evidence.7 8 9
In this week's BMJ, Finall and colleagues report a case of subcutaneous inflammation mimicking metastatic malignancy induced by injection of mistletoe.1 So how safe is this treatment? A wide range of serious adverse reactions have been noted, such as local reactions at the site of injection, anaphylaxis, dyspnoea, haemorrhagic colitis, herpes simplex, herpes zoster, joint pain, kidney failure, lymphangiitis, parasthesias, sarcoidosis, ulceration, and vertigo (Saller R. Zu den unerwuenschten Nebenwirkungen von Mistelpraeparaten. Drittens Mistelsymposium Otzenhausen, 20-22 November 2003).10
Findings from in vitro studies suggest that mistletoe extract may enhance the proliferation of some cancers.11 In addition, some patients with cancer may use mistletoe as an alternative to conventional treatments for cancer, rather than just a complementary treatment.
The claim frequently voiced by proponents of anthroposophic medicine—that mistletoe injections have no serious risks4—is therefore misleading.
Thus, mistletoe has been tested extensively as a treatment for cancer, but the most reliable randomised controlled trials fail to show benefit, and some reports show considerable potential for harm. The costs of regular mistletoe injections are high. I therefore recommend mistletoe as a Christmas decoration and for kissing under but not as an anticancer drug. At the risk of upsetting many proponents of alternative medicine, I also contend that intuition is no substitute for evidence.
Competing interests: None declared.