The content in leaves usually is lower. There are several dangers associated with the use of comfrey. One of the biggest problems is that people accidentally collect foxglove instead of comfrey. Foxglove is deadly. Among other things, it can cause irregular heart beats and convulsions.
PAs are hepatoxins and can cause irreversible liver damage.
One of the problems is that the effects of the alkaloids can be cumulative. Therefore, damage to the liver may not be associated to the alkaloids in comfrey. Also, the leaves and roots of comfrey have been shown to be carcinogenic. PAs from comfrey given to rats caused mortality. Liver pathology was characteristic of PA toxicosis.