Safety issues connected to the internal use of various Comfrey (Symphytum officinale, S. uplandicum x) species have been a controversial topic since I became active in the herb community in the early 1990s.
Many presentations in articles, blogs, classes and conferences prefer to address the 
therapeutic benefits of this plant and avoid directly giving information on the question of safety. The presentation usually has some version of “Do your own research and make your own decision” about comfrey. The main drawback to this approach is the reader – student is left with no guidelines on where or how to get accurate information.
The purpose of this article is to provide you with the best references I have come across over the many years of investigating Pyrrolizidine Alkaloids.
For those of you with neither the time nor desire to read these links, the short version is:
I recommend people to not use Comfrey internally.
Topical use as a poultice, salve, wash or herbal oil is fine.
Pyrrolizidine Alkaloid Research
Botany
Symphytum x uplandicum is also known as Russian comfrey.
It is a naturally occurring hybrid, being the result of S. officinale x S. asperum.
