Comfrey Herb: Danger or Not?
Comfrey is an herbal medicine with a history of efficacious use in humans. However, owing to the presence in comfrey of pyrrolizidine alkaloids (PAs), which are compounds known to be hepatotoxic, many countries have restricted its availability. This review emphasizes crucial aspects of PA toxicity, and suggests that comfrey might not be as dangerous to humans as current restrictions indicate
Comfrey herbal safety Rode, Dorena. Comfrey toxicity revisited. Pharmacol Sci. 2002 Nov;23(11):497-9.