In its traditional use to treat sprains and fractured bones, exposure to the toxic alkaloids would have been minimal. But comfrey became the wonder plant of the cosmetic industry for awhile. In the process of scientific rediscovery, it was also found to contain relatively high levels of protein, and on that basis was recommended for use as animal feed.
The presence of toxic alkaloids was discovered because some misguided people decided to use comfrey as the mainstay of their own diet... and were poisoned as a result. Since alkaloids can also be absorbed through the skin, its use as a daily cosmetic wasn't a good idea either - and so it was banned.
Comfrey can be invasive in rich soil, and will resprout from any small piece of root. I still have some small ones in the garden, so at some point I will find a place for them where they can grow but be contained, also far away from anything else harvested for root to prevent any error from happening - although the roots are quite distinctive, black skinned. Bumblebees absolutely adore the droopy pink flowers.
