Extracts of mistletoe (viscum album) are the most commonly used adjuvant to conventional cancer treatments in both Germany and Switzerland.
Mistletoe therapy was brought into the mainstream American consciousness after Suzanne Somers announced on Larry King that she would forego chemotherapy in lieu of Mistletoe therapy. Somers emphasized this was a personal decision and not one she necessarily advocated for others. Nonetheless, even conservatively speaking, her decision might not be as hasty as one might suspect. This is because it has been reported that her breast cancer was node negative and she agreed to be treated with a lumpectomy and radiation. Admittedly speaking without the complete details of her case, I think it's generally safe to say that her choice was not completely frivolous even from a mainstream perspective.
Perhaps the most extensively researched alternative therapy, with a number of clinical trials and many case reports, mistletoe extracts have demonstrated significant benefit for cancer patients in the majority of studies published.
The following is an excerpt from a 1990 report by the Office of Technology Assessments on Mistletoe:
Mistletoe has long been used in the treatment of a variety of acute and chronic conditions. It was not widely used for treating cancer, however, until the 1920s, during the early development of Anthroposophy, a modern 'spiritual science' applied to medicine and a variety of other disciplines. At present, mistletoe is given to patients either as the central component of a complex, broader treatment regimen in the practice of Anthroposophic medicine mainly in Europe or as a single agent partially or completely removed from the overall context of Anthroposophic care (e.g., in the United Kingdom and other countries). At present, mistletoe preparations are advocated mainly by Swiss and German physicians practicing Anthroposophic medicine, but are also used by other European physicians not necessarily associated with Anthroposophy. A larger group of researchers in Europe, and to a lesser extent in the United States, has focused on the study of mistletoe's biological properties in various experimental systems.
The following study examines the long term results of 1,668 cancer patients treated with mistletoe and compares them with over 8,000 matched patients, not treated with mistletoe. A significant benefit in terms of quality of life and survival is clearly demonstrated for breast, colon, rectum, stomach, lung cancers.
In August 2006 my good friend and colleague Dr. Simon Barker and I lectured on the use of mistletoe extracts in the treatment of cancer. We presented to the American Association of Naturopathic Physicians annual convention. Below is a link to the slides for the portion of the lecture I presented - which includes mechanisms of action and several human trials.