Comfrey is an herb used as a medicine and administered externally when treating such things as bruises, sprains, pulled muscles and ligaments, gum disease, canker sores, and sore throats. Some people use it internally for stomach aches or ulcers, but this can be very dangerous since comfrey contains substances called pyrrolizidine alkaloids (PA) which may cause liver damage or cancer. There have been reported deaths from eating or drinking comfrey remedies. Based on these facts, the Food and Drug Administration has recommended that comfrey be withdrawn from the market.