Scientific names: Viscum album L. (European mistletoe) and Phoradendron tomentosum (DC.) Engelm. ex A. Gray (Christmas mistletoe).

Common names: Mistletoe, bird lime, all heal, devil's fuge, golden bough, mistel (German), Iscador

Contraindications: Data is limited. Use of mistletoe extracts in patients with primary or secondary brain tumors, leukemia, or malignant lymphoma is contraindicated.

Pregnancy/nursing: Mistletoe contains toxic constituents. Avoid use during pregnancy or lactation.

Interactions: None well documented.

Side Effects: Local reactions following injection include redness, itching, inflammation, and induration at the injection site. Systemic reactions include mild fever or flu-like symptoms. Anaphylaxis has been reported.

Toxicities: Poison centers report toxicity of the whole plant, but especially mistletoe berries. The use of preparations standardized to small doses of ML-I or depleted of lectins may reduce toxicity.

Mistletoe is the common name for most obligate hemiparasitic plants in the order Santalales. Mistletoe grows on a wide variety of host trees such as pine, oak, birch, and apple. The term hemiparasitic is used to indicate that the mistletoe plant carries out photosynthesis independently but obtains water and minerals from the host. The name mistletoe originally referred to the species Viscum album (European mistletoe, of the family Santalaceae in the order Santalales); it was the only species native to Great Britain and much of Europe. A separate species, Viscum cruciatum, occurs in Southwest Spain and Southern Portugal, as well as North Africa, Australia and Asia. Over the centuries, the term has been broadened to include many other species of parasitic plants with similar habits, found in other parts of the world, that are classified in different genera and even families — such as the Misodendraceae and the Loranthaceae. In particular, the Eastern mistletoe native to North America, Phoradendron leucarpum, belongs to a distinct genus of the Santalaceae family. The genus Viscum is not native to North America, but Viscum album has been introduced to California. European mistletoe has smooth-edged, oval, evergreen leaves borne in pairs along the woody stem, and waxy, white berries that it bears in clusters of two to six. The Eastern mistletoe of North America is similar, but has shorter, broader leaves and longer clusters of 10 or more berries. The largest family of mistletoes, the Loranthaceae, has 73 genera and over 900 species. Subtropical and tropical climates have markedly more mistletoe species; Australia has 85, of which 71 are in Loranthaceae, and 14 in Santalaceae.

Mistletoe plants grow on a wide range of host trees; they commonly reduce their growth and a large plant stunts and commonly kills the distal portion of the branch it grows on. A heavy infestation may kill the entire host plant. Viscum album successfully parasitizes more than 200 tree and shrub species. Technically, all mistletoe species are hemiparasites, because they do perform at least a little photosynthesis for at least a short period of their life cycle. However, this is academic in some species whose contribution is very nearly zero. For example, some species, such as Viscum minimum, that parasitize succulents, commonly species of Cactaceae or Euphorbiaceae, grow largely within the host plant, with hardly more than the flower and fruit emerging. Once they have germinated and attached to the circulatory system of the host, their photosynthesis reduces so far that it becomes insignificant. Most of the Viscaceae bear evergreen leaves that photosynthesise effectively, and photosynthesis proceeds within their green, fleshy stems as well. Some species, such as "Viscum capense", are adapted to semi-arid conditions and their leaves are vestigial scales, hardly visible without detailed morphological investigation. Therefore their photosynthesis and transpiration only take place in their stems, limiting their demands on the host's supply of water, but also limiting their intake of carbon dioxide for photosynthesis. Accordingly their contribution to the host's metabolic balance becomes trivial and the idle parasite may become quite yellow as it grows, having practically given up photosynthesis.

At another extreme other species have vigorous green leaves. Not only do they photosynthesize actively, but a heavy infestation of mistletoe plants may take over whole host tree branches, sometimes killing practically the entire crown and replacing it with their own growth. In such a tree the host is relegated purely to the supply of water and mineral nutrients and the physical support of the trunk. Such a tree may survive as a Viscum community for years; it resembles a totally unknown species unless one examines it closely, because its foliage does not look like that of any tree.

A mistletoe seed germinates on the branch of a host tree or shrub, and in its early stages of development it is independent of its host. It commonly has two or even four embryos, each producing its hypocotyl, that grows towards the bark of the host under the influence of light and gravity, and potentially each forming a mistletoe plant in a clump. Possibly as an adaptation to assist in guiding the process of growing away from the light, the adhesive on the seed tends to darken the bark. On having made contact with the bark, the hypocotyl, with only a rudimentary scrap of root tissue at its tip penetrates it, a process that may take a year or more. In the mean time the plant is dependent on its own photosynthesis. Only after it reaches the host's conductive tissue can it begin to rely on the host for its needs. Later it forms a haustorium that penetrates the host tissue and takes water and nutrients from the host plant. Some species of the largest family, Loranthaceae, have small, insect-pollinated flowers (as with Santalaceae), but others have spectacularly showy, large, bird-pollinated flowers. Most mistletoe seeds are spread by birds that eat the 'seeds' (in actuality drupes). Quite a range of birds feed on them, of which the mistle thrush is the best-known in Europe, the Phainopepla in southwestern North America, and Dicaeum of Asia and Australia. Depending on the species of mistletoe and the species of bird, the seeds are regurgitated from the crop, excreted in their droppings, or stuck to the bill, from which the bird wipes it onto a suitable branch. The seeds are coated with a sticky material called viscin. Some viscin remains on the seed and when it touches a stem, it sticks tenaciously. The viscin soon hardens and attaches the seed firmly to its future host, where it germinates and its haustorium penetrates the sound bark. Specialist mistletoe eaters have adaptations that expedite the process; some pass the seeds through their unusually shaped digestive tracts so fast that a pause for defecation of the seeds is part of the feeding routine. Others have adapted patterns of feeding behavior; the bird grips the fruit in its bill and squeezes the sticky-coated seed out to the side. The seed sticks to the beak and the bird wipes it off onto the branch. Biochemically, viscin is a complex adhesive mix containing cellulosic strands and mucopolysaccharides. Once a mistletoe plant is established on its host, it usually is possible to save a valuable branch by pruning and judicious removal of the wood invaded by the haustorium, if the infection is caught early enough. Some species of mistletoe can regenerate if the pruning leaves any of the haustorium alive in the wood.

The most common species is technically known as European White-berried Mistletoe, Viscum album. Apple trees are one of Viscum album’s favourite hosts. There are many other mistletoes around the world, including some others used in Christmas and midwinter celebrations, but Europe’s Viscum album is the original and, arguably, the best mistletoe. Most popular traditions were originally based on this species, so it’s particularly important in folklore. It is the only mistletoe that has the distinctive forked branches, paired symmetrical evergreen leaves and pearlescent white berries associated with midwinter and Christmas.

Mistletoe preparations have been used medicinally in Europe for centuries to treat epilepsy, infertility, hypertension, and arthritis. The Celtic priests, known as Druids, revered the oak tree and the mistletoe that grew on it, according to Roman author and naturalist Gaius Plinius Secundus (also known as Pliny the Elder). At the winter celebration of Samhain, the sacred oaks were bare except for the green boughs of mistletoe, and this was taken as a sign of eternal fertility. The Celts placed a sprig of mistletoe above the door of their houses and its sacred nature prohibited fighting beneath it. This evolved over centuries into the custom of kissing underneath mistletoe at Christmas. In 1921, the Austrian anthroposophical spiritual leader Rudolf Steiner suggested that mistletoe might be used to treat cancer, based on the observation that mistletoe, like cancer, is parasitic and lethal to its host. Swiss and German clinics were founded to implement this idea and still actively use a mistletoe preparation fermented with a strain of Lactobacillus for 3 days. Mistletoe extracts contain several toxic proteins, several of which are lectins, or proteins capable of binding to specific sugars.

Mistletoe has been used to treat cancer, although there is a lack of quality clinical trials and no evidence of an effect. Most evidence is ancedotal and based on case reports. Further study is needed. In folk medicine, it has been used for its cardiovascular properties, but its clinical efficacy has not been established. Injectable mistletoe extract is widely used in Europe but is not licensed for use in the United States. Crude mistletoe fruit or herb is used to make a tea to treat hypertension at a dosage of 10 g/day. There are a number of proprietary extracts containing low levels of mistletoe lectin used as adjuvant cancer therapies. These extracts usually are given by intravenous or subcutaneous injection at dosages of 0.1 to 30 mg several times per week. Mistletoe preparations, produced according to anthroposophical methods, are given in incrementally increasing dosages depending on the patient's general condition and response to the injection. Use in pediatric patients has been reported. The pharmacokinetics in healthy adults has been determined.

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