Extract of a perennial herb that several studies have shown can have harmful properties when taken orally.
Topical application of comfrey has skin-soothing properties, but it is recommended only for short-term use and only then if you can be sure the amount of pyrrolizidine alkaloids is less than 100 micrograms per application—something that is impossible to determine without sophisticated testing equipment—making comfrey an ingredient to avoid. The alkaloid content means it has the potential to aggravate skin.
References for this information:
Chemical Research in Toxicology, November 2001, issue 11, pages 1546-1551