The harmful chemical in comfrey is called pyrrolizidine alkaloids (PA). This toxic substance can damage health, especially the liver, after ingestion. Comfrey is no longer sold in the U.S., except in creams or ointments. The United Kingdom, Australia, Canada, and Germany also have banned the sale of oral products containing comfrey. However, it seems that comfrey root is available for sale as tea in The Netherlands (https://voordeelkruiden.nl/smeerwortel).
Foraging, Growing and Using Herbs in Cities