Although comfrey was used to produce teas and infusions for hundreds of years, the presence of more than half a dozen pyrrolizidine alkaloids in the leaf and root pose a risk of liver toxicity to animals and people. The root, in fact, contains up to 16 times more pyrrolizidine alkaloids than the leaf.
The traditional use of comfrey in topical formulations, however, is supported by various tannins, rosmarinic acid and, most notably, a compound called allantoin.
The latter is used in the manufacture of toothpaste, shampoo, sun screen lotions and other cosmetic and pharmaceutical products.