Comfrey contains potentially dangerous compounds known as pyrrolizidine alkaloids. The roots contain higher levels of these compounds and mature leaves contain very little, if any, of these alkaloids.^{5, 6} Fresh young leaves contain higher amounts (up to 16 times more than mature leaves) and should be avoided.⁷ Other related forms, such as Russian comfrey (Symphytum uplandicum) and prickly comfrey (S. asperum), are sometimes available or mistakenly sold as regular comfrey but contain higher levels of these alkaloids.⁸ Several cases of people who developed liver disease or other serious problems from taking capsules or tea of comfrey have been reported over the years.⁹

Most comfrey products do not list their pyrrolizidine alkaloid content on the label. Therefore, it is best to avoid internal use of products made from comfrey root or young leaves altogether.