Symphytum officinalis the true species.
Everyone thinks they know comfrey and can identify this plant. What is not commonly known is that most comfrey plants seen in gardens is the agricultural hybrid Symphytum uplandicum hybrid. The botanical ID of this plant has been mightily confused due to the hybridisation program of creating the agricultural varieties.
This agricultural hybrid is actually much higher in the pyrrolizidine alkaloid (PA) compounds that have caused this plant to be restricted from internal use by the TGA (Therapeutic Goods Administration).
So to make a good comfrey product start with true species. I sourced seeds from Horizon herbs in Oregon United States. Then you harvest the leaves in preference to the roots and go for the older leaves rather than the young one. Dry the leaves then you have some raw material best suited for making medicine with a low PA content.
You can make tincture (for external usage only!) or an infused oil preparation.
A good reference for comfrey information is comfrey central. Google it. I especially recommend the article by Paul Berger who puts a good perspective on the do’s and don’ts of comfrey usage.