Comfrey is a herbaceous perennial native to Europe and temperate Asia. It is now naturalized in the United States. Also known as knitbone, comfrey has traditionally been used to treat superficial wounds as well as the inflammation of sprains and broken bones.

Allantoin, a key constituent found in the roots and leaves, is a cell proliferant that promotes wound healing and tissue regeneration. Comfrey leaf contains rosmarinic acid, which decreases inflammation and microvascular pulmonary injury. The mucilages in comfrey are responsible for its efficacy as a local demulcent (a slippery medicine for inflamed surfaces).

Although many traditional herbalists consider comfrey a beneficial herb, several pharmacological studies show that it is potentially quite toxic. Comfrey preparations, especially those taken internally, have been associated with veno-occlusive disease and even death. Many comfrey species contain poisonous pyrrolizidine alkaloids, compounds known to be highly toxic to the liver. Echimidine is one of the most toxic pyrrolizidine alkaloids. Dangerously high levels of this alkaloid have been found in both prickley comfrey (Symphytum asperum) and Russian comfrey (S. uplandicum). Although common comfrey (S. officinale) does not usually contain echimidine, other harmful pyrrolizidine alkaloids have been isolated from it. According to some research, small quantities of echimidine have been detected in about 25% of samples of common comfrey. Products made from the root should be used with extreme caution since the quantity of pyrrolizidine alkaloids in comfrey root is nearly ten times that in the leaves.

While S. officinale is the preferred source of comfrey herbal products, some comfrey preparations sold in the United States and Europe may unknowingly be made from Russian and prickley comfrey. Collectors who harvest raw plant material are not always able to identify different species of comfrey. Consequently, there are mislabeled comfrey products on the market that do not actually come from S. officinale, but instead come from other Symphytum species containing high echimidine levels. For this reason, special care must be taken in determining the source and botanical authenticity of commercial comfrey products.

Fond of moist soils, comfrey has an erect and stiff-haired stem, and it grows to a height of 20 to 120 cm. Its flowers are dull purple, violet, or whitish, and densely arranged in clusters. The wrinkly and hairy leaves are oblong, and often differ in appearance depending upon their position on the stem: the lower leaves are broad at the base and tapered at the ends while the upper leaves are broad throughout and narrowed only at the ends. The slimy roots show a horn-like appearance when dried. The root has a black exterior and fleshy whitish interior filled with a glutinous juice.

• Leaves
• Roots (rhizome)
• Aerial parts

Pyrrolizidine-type alkaloids (0.3%), including symphytine, symlandine, echimidine, intermidine, lycopsamine, myoscorpine, acetyllycopsamine, acetylintermidine, lasiocarpine, heliosupine, viridiflorine, echiumine; carbohydrates (gum, mucilage); pyrocatechol-type tannins (2.4%); triterpenes (phytosterols, steroidal saponins, isobauerenol); allantoin (0.75 to 2.55%), caffeic acid, carotene (0.63%), chlorogenic acid, choline, lithospermic acid, rosmarinic acid, silicic acid.

Comfrey ointments (containing 5% to 20% of the drug), creams, mucilaginous decoctions, poultices, and liniments are made from the fresh or dried aerial parts, fresh or dried leaf, and/or root of comfrey species and are used for external application. Experts advise using only products made from the mature leaves of S. officinale. Root preparations are available but are not recommended for either internal or external use. PA-free comfrey preparations are also available.

Traditional herbal actions: demulcent; internal and external vulnerary (wound healer), relaxing expectorant, astringent, emollient, pectoral tonic.

Comfrey preparations are used externally (only on intact skin) for bruising, pulled muscles and ligaments, sprains, and blunt injuries. The root has anti-inflammatory, callus-promoting, and antimitotic action.

Clinical applications: Comfrey is a stimulant to fibroblast, osteoblast, and chrondroblast activity, and is thus used for fractures, sprains, and strains. The pyrrolizidine alkaloid–free form is used internally for ulceration or erosion of the gut wall and congestive bronchial conditions.

In in vivo studies, unsaturated pyrrolizidine alkaloids isolated from comfrey have been shown to have hepatotoxic, carcinogenic, and mutagenic effects. In experiments in animals, comfrey extract given orally to rats had wound healing and analgesic properties and stimulated the actions of the drug-metabolizing enzyme, aminopyrine N-demethylase in the liver. In in vitro research, comfrey extract increased uterine tone.

Rosmarinic acid isolated from comfrey is responsible for in vitro anti-inflammatory effects. In other studies, pyrrolic esters found in comfrey exhibited weak antimuscarinic effects in clinical trials. Sarracine and platyphylline are nonhepatotoxic pyrrolizidine alkaloids used clinically to treat gastrointestinal hypermotility and peptic ulcers in human patients.

Herb and leaf ointments—root extracts and semi-solid products for external use: 5% to 20% dried drug or equivalent preparation. Daily dosage should not exceed 100 mcg of pyrrolizidine alklaoids with 1,2-unsaturated necine structure, including their N-oxides, for more than four to six weeks per year. Comfrey is also available as a 1:5 tincture of root and leaves (PA-free tincture, 1 to 2 ml tid). Capsules are not currently available due to concerns about PA content.

Comfrey is basically safe if taken within recommended therapeutic dosages and if used only as external preparations on unbroken skin. However, internal consumption of comfrey over prolonged periods of time may cause hepatic veno-occlusive disease. Oral intake of comfrey is also associated with several cases of atropine poisoning. However, reported cases of atropine toxicity may have been due to deadly nightshade (belladonna leaf) contaminants in raw plant material used to manufacture comfrey products. Contamination can occur because comfrey and belladonna leaves resemble each other.

Topical comfrey preparations should never be used on broken skin. Pregnant and lactating women should not use comfrey products under any circumstances.

No clinically significant interactions between comfrey and conventional medications are known to have been reported in the literature to date, including the German Commission E monograph, which represents years of extensive review of this herb (Blumenthal 1998).

Comfrey products are sold in the United Kingdom, France, and Germany as nonprescription drugs. The U.S. FDA classifies common comfrey as a dietary supplement.

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