I have searched through as much as I can, but can't find any reference to my specific question.
Does anyone know about potential links between applying comfrey oil topically and cancer. Apparently some health food shops are advising clients that they should only apply comfrey oil for 10 days at a time as it can cause skin cancer. I have checked safety data sheets and done as much checking around on websites as I can, but can't find any mention of this link.
In all the discussions I have read or been part of over the past twelve years or so, I have never seen any mention of comfrey causing skin cancer. (That doesn't mean there hasn't been any allegations, it means I haven't heard them!) Comfrey does help skin cells regenerate, which is why someone may have got the wrong end of the stick and thought that because it does that, it could cause skin cancer. You do have to be very careful not to apply comfrey to an open wound which has not been properly cleansed, because the new skin growth could trap any nasties inside which would then become inflamed and could become infected.
If you want an authority on the subject, I would suggest you go to Paul Bergner's site or even sent him an email with the specific question (email@example.com. He's done a lot of work on comfrey and liver toxic properties and is very highly regarded.