However, it has toxic substances called pyrrolizidine alkaloids that damage the liver and can lead to death. Comfrey is no longer sold in the U.S., except in creams or ointments. The United Kingdom, Australia, Canada, and Germany also have bannedthe sale of oral products containing comfrey
From page 4 of the link I provided.
From Martin Jaegar:
"All this controversy started because a guy in Australia decided to self-medicate using Comfrey, and could not be bothered to research it enough to know what he was doing. Basically he brewed up a cup of Comfrey tea, using couple teaspoons of the root each time, and drunk two or three cups every day, day after day, week after week, month after month, until the pyrrolizidine alkaloids (symphytine, echimidine, symglandine and lycopsamine) built up enough to poison his liver. I don't recall how many months that took to kill him...I'd guess six months to a year. So then the Aussie equivalent of our FDA banned Comfrey as a deadly substance, and not to be outdone, our beloved FDA tried to do the same. It would seem neither understands that everything has an overdose potential, and everything that has a good effect also has a bad effect. Chug a big bottle of Bacardi 151 Rum, and you die. Take too many AMA-sponsored, FDA-approved, doctor-prescribed sleeping pills, and you die. Anyway, using comfrey leaf poultice, salve, etc., is not dangerous. Comfrey leaf tea is not dangerous if taken sensibly. Comfrey root should only be used by a actual herbalist that knows what he is doing."
Should be noted also that the root is ten times as powerful as the leaves.