Several studies have shown that comfrey extract can have carcinogenic or toxic properties when taken orally. It is a major problem for the body when consumed orally because of pyrrolizidine alkaloids. These compounds occur naturally in every part of the comfrey plant, and are absorbed through the skin, where they cause problems when the liver attempts to metabolize them. It is these metabolites (referred to as pyrroles) that are highly toxic (Sources: www.naturaldatabase.com; International Journal of Molecular Sciences
, 2002, pages 948–964; and http://www.ansci.cornell.edu/plants/toxicagents/alkaloids/pyrrolizidine.html
Topical application of comfrey has anti-inflammatory properties, but 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 would be impossible to determine without sophisticated testing equipment, making comfrey an ingredient to avoid. The alkaloid content makes it a potential skin irritant (Sources: Chemical Research in Toxicology, November 2001, pages 1546–1551; and Public Health Nutrition, December 2000, pages 501–508).