The main liver injury caused by comfrey (Symphytum officinale) is veno-occlusive disease, a non-thrombotic obliteration of small hepatic veins leading to cirrhosis and eventually liver failure. Patients may present with either acute or chronic clinical signs with portal hypertension, hepatomegaly and abdominal pain as the main features. Therapeutic approaches include avoiding intake and, if hepatic failure is imminent, liver transplantation.
– Public Health Nutr 2000 Dec;3(4A):501-8 — The efficacy and safety of comfrey. — Stickel F, Seitz HK.

Hepatotoxicity due to comfrey herb tea.
– Hepatotoxicity due to comfrey herb tea. — Ridker PN, et al.
Am J Med. 1989 Dec;87(6):701

The regular use of comfrey as part of the diet or for medicinal purposes may be a potential health risk as a result of the presence of naturally-occurring pyrrolizidine alkaloids. The majority of these alkaloids are hepatotoxic in both animals and humans, and some have been shown to induce tumours in experimental animals.
– Med J Aust 1988 Dec 5-19;149(11-12):678-82 — Comfrey: assessing the low-dose health risk. — Abbott PJ. — Toxicology Unit, Department of Community Services and Health, Canberra, ACT.