Mistletoe is the English common name for over 1200 species of a semi-parasitic plant that grows on trees worldwide, though only viscum album (European or common mistletoe) is used in the treatment of cancer and inflammatory conditions. References to mistletoe as a sacred plant and remedy appear throughout history and date as far back as the writings of Hippocrates (c.460–c.370) and Pliny the Elder (AD 23–79), but it was only through the indications of Rudolf Steiner, in the early 1920's, that it became known that mistletoe could be used for the treatment of cancer. It was then developed as an injection.

Since that time, European oncologists have been using the liquid extract of the mistletoe plant to improve survival in patients with cancer and to improve quality of life by reducing the side-effects of chemotherapy and radiation, such as nausea, vomiting and lack of appetite, as well as diminishing tumor-related pain. Mistletoe injections are currently among the most widely used complementary cancer treatments in Europe. It is approved for palliative care use in several European countries, but not yet in the United States. Until further clinical testing is done in the US, oncologists cannot offer this treatment as standard of care for cancer patients. Mistletoe is being administered in the US by complementary medicine practitioners, but mistletoe is not approved by the FDA for cancer treatment.

The clinical effectiveness of mistletoe extracts in cancer has been investigated in a great number of studies and clinical trials, primarily in Europe. A Phase I clinical trial in the US recently concluded at Johns Hopkins School of Medicine.