Symphytum officinale, in the same family as borage and forget-me-not, has a long history of use in folk medicine. The common form, native to Europe, is not desirable in the garden, being quite invasive in moist, temperate environments. It is formally designated as invasive only in Oregon and Alaska but has naturalized in most of the states bordering Canada. The roots are deep and difficult to dig out and it readily propagates from both seeds and root fragments. A sterile hybrid, S. x uplandicum or Russian comfrey, is advertised as less invasive but its roots are still a problem.
As with most botanicals, comfrey has been recommended for a variety of ills. Gerard’s Herbal reports its benefits “for those that spit blood”, and combined with other herbs, “a most excellent medicine against a gonorrhoea or running of the reins”. However, it has been most consistently promoted for external use to assist healing of injuries, including wounds, bruises, sprains and fractures, as suggested by popular names like knitbone and bruise wort.
In the two decades since I began writing this series the quality of research on complementary medicine has improved. Randomized clinical trials with appropriate controls and effective blinding are available for many herbal remedies. However, they are still impaired by the heterogeneity of the products, which may contain several potentially effective agents that are present in different concentrations in various parts of the plant, and which may vary with growing conditions. Because of limited funding, many of the studies are underpowered with too few subjects. In the case of comfrey, a well-designed study of 220 patients with painful osteoarthritis of the knee were treated with a comfrey root ointment or with a placebo. The active treatment group showed significantly greater pain reduction, joint mobility and improved quality of life.
So, it seems that there is some evidence that comfrey may be beneficial when applied externally. A tea brewed from the leaves, or powdered root, has also been long used in traditional medicine. Though they seem to be rare, cases of severe and sometimes fatal liver damage have been reported after ingestion though they have been associated with large quantities and prolonged use. Animal studies have confirmed the toxicity which is due to a family of substances called pyrrolizidine alkaloids (PAs). In rodents PAs can produce both liver damage and a variety of tumours. Canada and Germany have banned comfrey-containing products intended for internal use. In 2001 the FDA issued an “Advisory” against the internal use of comfrey, and against its external application to open wounds. Products suitable for the brewing of comfrey tea are still available in this country, as a trip to the Amazon website will readily confirm. For example, a seller of a one-pound package of leaf advises against its internal use but the reviews show that many users are regularly drinking it as a tea.
For the gardener comfrey also has its uses. If you half fill a bucket with leaves and top it up with water, in a few weeks you are promised a “dark, foul-smelling manure tea” that provides a wonderful side-dressing for plants, especially tomatoes.
Most of the beneficial effect of comfrey is attributed to allantoin, with some help from rosmarinic acid and tannins. Many websites promoting the healing effects of this agent mention that chamomile and sugar beet are also good sources. What they don’t mention is that allantoin is found in the urine of most mammals, with the exception of man, the anthropoid apes and ‘the Dalmatian coach hound’. It is the end product of the metabolism of purines released by the breakdown of DNA and RNA after cell death. Man, and the other animals mentioned, lack the enzyme that converts uric acid to allantoin.
The toxic PAs are more concentrated in the root than in the leaves, and the risk of drinking an occasional cup of comfrey leaf tea is probably small. Though the therapeutic possibilities of comfrey would hardly seem to justify such an effort, two laboratories in Germany have used modern genetic engineering techniques, RNA interference and CRISPR, to produce “hairy root mutants” deficient in an enzyme required for the synthesis of PAs. It is questionable whether those seeking a ‘natural’ soothing beverage for their aches and pains will be attracted to a GMO version of the tea.