The Walden Effect: Farming, simple living, permaculture, and invention.

Is comfrey carcinogenic?

Russian comfrey flowers

I seem to have opened a can of worms with my words: "Don't take comfrey internally.  According to some sources, comfrey can be a carcinogen, so I figure it's better not to risk it."  Some of our readers feel that comfrey is an herb that's been used for hundreds of years and is safe by default, while others became concerned that maybe the leaves aren't even safe enough to feed to their livestock (and that problems might pass on to humans if we eat meat or eggs from animals that consumed comfrey).

Making mint teaI should admit up front that I hadn't done much research on that assertion before throwing it into a post.  I grew up drinking comfrey-mint tea that my mom made by clipping comfrey and mint leaves, putting them in a mason jar, pouring hot water over top, then refrigerating.  After we drank the iced beverage, I chewed up the greenery and was well satisfied.

However, during my teenage, rebellious years, I started questioning things.  This was before I used the internet, so I'm not sure what book I found the data in, but some source told me comfrey leaves can be carcinogenic.  I passed the data on to Mom, who changed her recipe to mint only, and my rebellion was over.

Since many of you seemed worried about the topic, I decided it was high time to find out the truth, which meant delving into the scientific literature.  The issue is turns out to be that comfrey contains pyrrolizidine alkaloids, chemicals that have caused liver cancer in rats at various doses.  In one study, cancer was induced by feeding rats comfrey leaves for 1.5 to 2 years.  In another study, the extracted alkaloids were injected into the rats at a rate of 20 mg/kg three times a week and caused liver damage within 18 weeks.

One of our readers sent me to this refutation of comfrey problems, so I checked that out next.  However, even my quick search of the internet turns up problems with their refutation.  For example, leaves instead of roots were used in the studies I saw, and at least one study consisted of animals eating comfrey leaves instead of being injected with extracted chemicals.  In addition, that website only looks at common comfrey, while many permaculturalists are using Russian comfrey in the same ways.  Finally, it's worth noting that the author clearly has a financial interest in making comfrey available as an herb.

I don't have any solid conclusions since I think we each have to make health decisions for our own families and animals.  I still am unlikely to take comfrey internally, am quite happy to apply it externally, and will keep feeding the leaves to chickens since our working birds have a shelf life of 1.5 years or less.  If we ever owned breeding or milk animals that I wanted to really go the distance, though, I might keep them away from comfrey, and I wouldn't eat any animal's liver if it looked weird.  I hope that clears, rather than muddies, the waters!

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About us: Anna Hess and Mark Hamilton spent over a decade living self-sufficiently in the mountains of Virginia before moving north to start over from scratch in the foothills of Ohio. They've experimented with permaculture, no-till gardening, trailersteading, home-based microbusinesses and much more, writing about their adventures in both blogs and books.

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Anna, if you were assigned to defend comfrey noncarcinogenic do you think you could write a good case? PS your publishing house was interviewed on NPR last night, Skyhorse. Made me a bit proud.
Comment by Maggie Tue Nov 19 08:11:06 2013

In a hurry, so haven't checked on the variety (my family stock has white flowers), but you do know the tale, Anna, of your maternal great-grandmother's tea mint-comfrey, till her death at 101? I think Aunt Ruth picked only the tenderest comfrey leaves, as we came to hear that the old ones were cancer-causing, but the baby ones were OK (tho how this could be is beyond me!)

I missed eating those baby leaves after I gave them up--only okra comes near to their slimy, bland (and cooling) texture! I wonder what Chinese and Hindu medicine says about comfrey!

Comment by adrianne Tue Nov 19 08:12:30 2013

I love to see comfrey posts because the very first post I ever saw on your blog was the aforementioned feeding comrey to livestock post.

You can definitely see that your chickens will eat what they want to and avoid the rest. It might be a matter of giving access to toxic plants, like Sepp Holzer advises, and letting the animals take only what they will.

As long as we don't force hungry animals to fill up on what they don't want, they'll probably stop when their bodies tell them to.

Comment by Matthew Tue Nov 19 09:30:42 2013

I'm always skeptical of anything causing cancer in rats, mostly because first off, isn't cancer hereditary? Who's to say that the rats weren't predisposed? Second, they always feed or inject more than any reasonable person is going to use or ingest. If you force fed a human being their weight in Equal I bet they'd get cancer, too.

There is an equal controversy about poke weed. My grandparents grew up eating it as kids, (the tender young stems, not the poisonous leaves and berries), but studies show that it is dangerous? Again, you have to eat a ton of it to get sick, while the actual period in which poke is "ripe" is two or three short weeks in the very early spring. You'd be lucky to eat a pound all year, and the studies show you have to eat several pounds to get sick. By next years poke season, whatever small amount of poisons you ate last year are long gone.

That's a long way to say I'm sure small amounts of comfrey are probably fine. Too much of anything is a bad thing.

Comment by Emily from Bristol Tue Nov 19 11:49:38 2013

With regard to comfrey and other plants that contain pyrrolizidine alkaloids (PAs) like retronecine, I would worry more about liver damage than them being carcinogenic. Damaging your liver is an effective if unpleasant way of doing yourself in. Historically, acute liver failure has a mortality of over 80%. In which case you won't have time to develop cancer!

A basic principle of toxocology is that the dose makes the poison. Since the concentration of PAs is bound to vary from species to species and from plant to plant, it will be difficult to say how much of a plant is safe. But reading this story, it seems that even harvesting grains mixed with weeds containing PAs can be harmful. In 1920 wheat flour contaminated with plants containing PAs resulted in an outbreak of "bread poisoning" (PDF file). This also indicated that e.g. boiling is not enough to destroy PA's like retronecine.

As you can read from the linked article on PAs, livestock (especially ruminants) seems to be able to handle them better then humans, possibly due to their different digestive system.

@Adrienne: Unfortunately the anecdote of a great-grandmother living to the ripe age of 101 while drinking mint-comfrey is not evidence one way or another. Because of the fallacy of incomplete evidence.

@Emily: Cancer is not really hereditary in the true sense of the word. But as the linked page describes;

some people inherit faulty (mutated) genes which make their chance of developing a particular cancer higher than normal.


Up to 1 in 20 (5%) cancers are caused by an inherited faulty gene.

Which implies that a large majority are not.

Comment by Roland_Smith Tue Nov 19 15:52:05 2013

I use comfrey occasionally. I drink comfrey tea once in a while. I use different herbs for different needs and have one abiding rule: if it tastes good to me i drink it, if not i don't. Our bodies know. Animals know better than we. Local cows break out of the fencing to eat Burdock when needed. Back to moderation and healthy living has helped me recover from many health challenges; two time cancer/hep b and c/rheumatoid arthritis/recovered heroin addict and sober alcoholic with the help of herbal remedies, growing our food and staying away from auto exhaust. Of greater concern to me and my liver is the fact that the area where i live was opened by silver mining and has a level of arsnic in the ponds and lakes that is six times the level allowable for human consumption. Comfrey is the least of our worries, eh! All the best from the bush...

Comment by Linda Roberts Wed Nov 20 19:30:30 2013

if it tastes good to me i drink it, if not i don't.

Unfortunately taste or smell is not a reliable indicator of toxicity. E.g. the death cap mushroom (Amanita phalloides) is reported to taste very nice even though it is actually very poisonous.

Our bodies know.

It that were true, would e.g. food poisoning exist?

Of greater concern to me and my liver is the fact that the area where i live was opened by silver mining and has a level of arsnic in the ponds and lakes that is six times the level allowable for human consumption.

That is indeed a matter for concern, especially since it is probably anorganic arsenic compounds, which are more toxic than organic arsenic compounds. But with chronic low-level exposure, heart disease looks like a bigger risk than liver toxicity. It seems that eating garlic can help, though.

Comment by Roland_Smith Thu Nov 21 01:26:41 2013

Actually a bad taste is not an indication of a "poisonous" plant. Bitter Herbs are quite good for you. Horehound is nasty tasting along with a lot of others. Tree barks are very bitter.

Swedish Bitters is a wonderful Tonic.

A plant is only "poisonous" if you take to much. The Study of Herbal Medicine is a must to understand the uses, and dosages. I mean old Natural Medicine Books...not New Age books which mixes health with spiritualism.

Plants were created for medicine, and meat (food).

Genesis(1:30) And to every beast of the earth, and to every fowl of the air, and to every thing that creepeth upon the earth, wherein there is life, I have given every green herb for meat: and it was so.

Ezekiel(47:12) And by the river upon the bank thereof, on this side and on that side, shall grow all trees for meat, whose leaf shall not fade, neither shall the fruit thereof be consumed: it shall bring forth new fruit according to his months, because their waters they issued out of the sanctuary: and the fruit thereof shall be for meat, and the leaf thereof for medicine.

Our food is supposed to be our medicine. This is not so today since the food supply has been compromised.


Comment by Edith Thu Nov 21 13:44:13 2013
the comfrey may be one of those plants that cleans the soil of toxins and you don't want to eat those from a toxic site
Comment by Anonymous Sun Nov 8 12:44:42 2015

How does Dr. Chris have a financial interest in a herb thats pretty much free? Even if he sold it himself he's not making a living off of it. You can buy it by the ounce for as low as a couple dollars. I find more problems with the actual scientific literature, who the heck would ever isolate the harmful compound found in Comfrey dose it up and then inject it under the skin of mice or let alone people? Who also would feed any species solely one food type let alone a medicinal plant (I read it was actually 33% of their diet and they failed to mention other variables and conditions, like that they were probably inbred mice.)

They banned it because they don't want people eating it and figuring out its medical secrets. Like the ability it has to regrow limbs. I'm eating it as we speak.

Comment by Brandon God-Conscious Thu May 5 23:10:08 2016

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