(Comfrey contains hepatotoxic pyrrolizidine alkaloids. More info here: Livertoxic PAs --Henriette.)
466. SYMPHYTUM.—COMFREY. The root of Sym'phytum officina'le Linné. Habitat: United States and Europe; cultivated. About 150 mm. (6 in.) or more long, and from the thickness of a quill up to an inch in diameter, often split; externally black, wrinkled; internally whitish, and horny when dry; inodorous; taste sweetish, astringent, and very mucilaginous, containing as much mucilage as, or more than, althaea, for which it may often be substituted. It is chiefly used as a demulcent in domestic cough remedies, and has been highly esteemed as a vulnerary. Dose: 2 to 4 dr. (8 to 15 Gm.).
A Manual of Organic Materia Medica and Pharmacognosy, 1917, was written by Lucius E. Sayre, B.S. Ph. M.