Comfrey is the victim of a bad press, inaccurate reports, and four true cases of toxicity which in themselves are not straightforward, but suggest overdosing on comfrey. Governments in the UK & Australia have restricted the uses of comfrey root or banned the plant respectively.
The problem is two-fold: firstly there are two "comfreys" and reference to them is often casual. Regular, common, medicinal comfrey is Symphytum officinale. Russian comfrey, the great compost heap maker, is Symphytum x uplandicum. Medical herbalists in the UK, from whose written reports I am extrapolating, point out that Russian comfrey was probably the herb used in the toxicity trials yet regular comfrey is also restricted or banned.
Secondly, when the toxicity tests were done in the late 70s, a chemical constituent called pyrrolizidine alkaloid was isolated, extracted from <comfrey> leaves & injected into baby rats at what many medical herbalists consider an "unrealistic level". In other words far more comfrey than a human would eat to get such a toxic level of <PAs>. Also baby rats are smaller than humans; they do not have the same metabolism as humans; and an isolated chemical injected outside the rat's stomach wall is not the same as a human eating leaves with many chemical constituents and digesting them normally. A chemical in isolation will cause different reactions from a group of chemical constituents containing that one as well.
To digress, but to explain, I hope. Aspirin is a synthesized chemical, acetylsalicylic acid, based on a real life plant constituent found in meadowsweet & willow. Aspirin can cause ulcerations of the stomach lining; meadowsweet has a soothing, gummy constituent called mucilage which lines the stomach, preventing erosion of the stomach wall but allowing the anti inflammatory properties of the salicylates of the herb to be utilized. OK?
So -- the bad guys in <comfrey>, the <PAs> were isolated & did bad things. But that too must be qualified.
The early research, late 70s, concluded that these <PAs> do indeed cause liver damage in humans. Medical herbalists would point out that Pyrrolizidine alkaloids can cause obstructions of the veins in the human liver, known as hepatic veno-occlusion, but <were not shown to cause liver cell abnormalities> and that the level of alkaloids in comfrey was too low to <cause specific damage to liver circulation> in any case.
And finally, is comfrey carcinogenic? The carcinogenic alkaloid has been identified as symphytine which apparently is about 5% of the total alkaloids in comfrey.
The original, often cited report was written by Culver et al in 1980. There have been many criticisms since of the research itself; how the scientific testing was conducted, which comfrey was really used, etc. What I found most interesting was the tumors in all but three of the rats were benign -- out of three groups of 19-28 rats and 3 groups of 15-24 rats. <And the three malignant tumors were of low malignancy>. There were clear cut cases of liver damage. That's in rats.
There are four cases involving humans which do implicate comfrey. One involved a woman who was finally diagnosed as having veno-occlusive disease & did consume a quart of herbal tea/per day that contained comfrey. A second case involved a boy with Crohns disease who was treated with conventional medicine for some time before going over to comfrey root & acupuncture. The long running malnutrition may have weaken the liver predisposing it to the venal obstruction problem. Comfrey root was blamed. The drugs were not considered as possibilities. The third case involves a woman who overdosed: 10 cups of comfrey tea a day & handsful of comfrey pills. After 9 years, she had serious liver problems. The fourth case became a fatality. A vegetarian, given to specific food binges for weeks, took an unknown amount of comfrey for flu like symptoms possibly over a period of four months. The particulars of his case are blurred. All cases involve comfrey; in at least three, there are suggestions of overdose or abuse of the plant. WHICH plant, I don't know.
There are also disagreements about the efficacy & safety of leaves vs. root. Some studies show the leaf to be almost alkaloid free -- thus safe. The UK finally restricted the internal use of comfrey root... saying that there are still too many unanswered questions. Most medical herbalists I know will politely to vigorously disagree, but the law restricts the root. At least externally the root's OK here & the leaves can still be used as tea or poultice.
I'm sorry this is so long, but bear with me one more paragraph, please. I must credit Penelope Ody, MNIMH, former Editor, writing in Herbs, the British Herb Society magazine & Margaret Whitelegg, MNIMH, whose paper for the National Institute of Medical herbalists to the UK government in <Defence of Comfrey> was later published in the European Journal of Herbal Medicine. Both were published in 1993. I cannot do justice to their articles so briefly, but I do hope I have fairly summarized their writings. Any misstatements, confusion of explanations here are mine.
Comment from Henriette:
Yes, the pyrrolizidine alkaloids (PAs) in comfrey do hurt your liver. Yes, you have to take lots of the herb in order to get veno-occlusive liver disease. No, you can't blame that disease on pharmaceuticals taken at the same time - they'd hurt the liver in _other_ ways. So don't take lots of comfrey every day for weeks at a time; if you do believe that you need it (and not, say, Calendula, which works much the same in wounds, or Plantago, which works much the same way both in wounds and in coughs; neither of these are problematic), take it in small amounts.
Know that if your liver is healthy it'll get hit worse than if it's already compromised. That is because the hepatotoxic PAs are catalysts, much like freons in the ozone layer - each cell tries its best to detoxify this molecule, can't do it, dies, and the next one tries, until the PA is passed out unchanged.
And know, too, that PAs are absorbed through the skin. That means that it's a really bad idea to use comfrey long-term for wounds.
Some comfreys are more toxic than others. Russian comfrey (Symphytum x uplandicum) is one of the worst. And there's more hepatotoxic PAs in comfreys that are grown without a real winter, eg. in California.
Other plants contain the same kinds of hepatotoxic PAs. Among these are, for instance, borage (not in the seeds, not much in the flower), some of the senecios, germander (Teucrium chamaedrys), and lungwort (Pulmonaria sp.). The most toxic ones of the lot are the Lithospermums.