I looked up "comfrey" on google because I wanted to know what it looks
like. Curiously, in the first couple of links i found sites promoting
its use as a digestive aid and for wound healing, and sites warning
about how very toxic the plant is, and how you shouldn't ingest it or
expose broken skin to it. I gather substances in it are toxic to the
liver and promote cancers. (Of course, promoting cell growth might
both promote cancer and also promote wound healing, I suppose.)