By now you may be thinking this herb is too good to be true. There has to be some side effect, you might be thinking. Even though comfrey has long been esteemed as a vulnerary the medieval term for a plant used to heal battle wounds. It has come under fierce ridicule in recent years because of the discovery of the pyrrolizidine alkaloids. These are believed to be hepatoxic (toxic to the liver). Using comfrey externally as a poultice or as a compress has always been safe. However recent scientific studies indicate that comfrey, when ingested in large amounts and over long periods of time will cause liver damage. This is because of the pyrrolizidine alkaloids it contains. These pyrrolizidine alkaloids occur in a number of other plants as well, including ragwort, borage, and coltsfoot. In the 1980s there was a research paper that reported liver damage in laboratory animals that had varying doses of these alkaloids injected into them intravenously. This came as shock to the Herbalist community because comfrey has long been regarded as one of the safest herbs.
This contra-indication is primarily for individuals with inherently weak livers, or predisposition for liver disease, such as alcoholics, etc. When at-risk individuals take comfrey in large amounts and over long periods of time, there is a chance that more damage will occur to their already weakened liver. People who take liver compromising medication are also at risk when ingesting comfrey. The pyrrolizidine alkaloids will only compound an overloaded liver’s problems because it has been busy filtering the synthetic drugs. Consequently, disease sets into the liver or it begins to shut down after being overloaded with more filtrate than it can remove.
Naturally, experts rushed to the defense of comfrey. One expert pointed out that the rats had been fed the equivalent of twenty-four times their body weight in comfrey leaves. (Green Pharmacy Barbara Griggs 305) Fred Fletcher Hyde argued forcefully that a plant is not only a physical dilution of its chemical constituents:
Teas, almonds, apples, pears, mustard radishes, and hops, to list only a few items, all contain substances which, if extracted, can be shown to be poisonous when tested under conditions similar to those used in the comfrey experiments. Must we then ignore our experience of the usefulness and wholesomeness of these foods because controlled trials and scientific evidence have not been published to establish their safety? (Green Pharmacy Barbara Griggs 305)
Other data supporting the relatively low toxicity of comfrey was published in the journal of Science by noted biochemist Bruce Ames, Ph.D., of the University of California at Berkeley, indicates that comfrey leaf tea is less carcinogenic than an equivalent amount of beer. (The Green Pharmacy by James A. Duke, Ph.D. 77) As a result of the study on rats, comfrey has been banned from sale in many countries including Canada, Australia, and Japan. (Nutritional Herbology by Mark Pederson 73)The countries that have not banned comfrey completely have limited it to external uses.
The long and short of the controversy about comfrey’s pyrrolizidine alkaloids is that they can compound anyone’s preexisting liver problem if it is ingested for too long. Therefore extended internal use of comfrey should be avoided.Children, pregnant women, and nursing women should avoid the ingestion of comfrey as well. I recommend keeping in mind the following ideas when considering the ingestion of comfrey: Because of the nature of comfrey (its fast healing action), one never needs to take it for a considerable length of time. As a precaution, cleanse your liver before, during and after ingesting comfrey. I recommend taking milk thistle as a supplement along with comfrey to protect your liver. This will offset any adverse effects that comfrey’s pyrrolizidine alkaloids may cause.