Response to #3
3. Studies were performed by injecting plant constituents under the skin or in tissue cultures, exhibiting an unrealistic scenario of this plant's use.

Planta Med 1989 Dec;55(6):518-22
Studies on the effect of alkaloid extract of Symphytum officinale on human lymphocyte cultures.
This study used an isolated extract of comfrey on human lymphocyte cultures to show toxicity.

This study fails to demonstrate the whole herb's toxicity and was not administered as a decoction or infusion (the way comfrey is realistically consumed)

J Natl Cancer Inst 1979 Aug;63(2):469-72
Injection of hepatic tumors in rats by senkirkine and symphytine.
This study was conducted on 3 groups of 20 rats group 1 injected with an extract of senkirkine, group 2 with symphytine (an alkaloid of comfrey), and group 3 with a NaCl solution. The rats were predisposed to liver problems (male inbred ACI rats). All group 1 rats survived for more than 290 days after the start of injections, and 9 of 20 rats developed liver cell adenoma. All group 2 animals survived for more than 330 days after the start of injections. Of 20 rats 4 had liver tumors, 3 had hemangioendothelial sarcomas, and 1 had liver cell adenoma. The control group 3, had no liver tumors.

This test demonstrated that injected isolated constituents may be harmful. However, it failed to show the toxicity of the whole plant administered realistically.

Hirono I Mori H, Haga M: J Natl Cancer Inst. 61(3):865-868
Carcinogenic activity of Symphytum officinale.
Rats were injected with isolated alkaloids from Symphytum o. resulting in mild liver damage compared to a control group.

This study proved that in order to produce even a limited amount of liver damage in a rat it is necessary to administer the equivalent of several times its own body weight of comfrey leaf (when the results are expressed as fresh material) or at least 1% of its diet as comfrey root. It is difficult to envision a situation in which a human subject would assimilate an equivalent amount of comfrey over a similar time span. Deaths in the rats occurred at levels which for humans would be equivalent to the alkaloid from 19,880 leaves or 28 times his body weight. So to extrapolate the results of Hirono's study to humans, it would be necessary to eat three ounces of fresh comfrey leaf every day for 56 years to induce any liver damage.

T. Furuya and K. Araki 1968 Chem. Pharm. Bull 16 2512-1516
Studies on constituents of crude drugs specifically alkaloids of Symphytum officinale
Pharmacological tests with rats showed symphytine to have a LD
50 of about 300 mg/kg or rat tissue when given intravenously.

At this level the consumer could never attain the lethal dose of 300 mg/per kg of tissue found necessary to produce the acute reaction in rats.
Injecting plant constituents does not have the same physiological effect on the body as drinking an infusion (tea) of the plant.
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