2. Studies were performed with extremely concentrated extracts or single constituents of the herb containing higher Pyrrolizidine alkaloid content than the whole plant.
Nat Toxins 1996;4(4):163-7
Analysis, separation, and bioassay of pyrrolizidine alkaloids from comfrey (Symphytum officinale).
Demonstrates that some parts of the plant contain higher pyrrolizidine content than others (the leaf containing the lowest). And that the effect of the alkaloids individually is different from that of the alkaloids in the whole plant extract.
Alkaloids react differently in the body when they are isolated. Many studies are done on the effect of these isolated constituents, but the results cannot be used to determine safety of the whole plant.
Int J Exp Pathol 1993 Apr;74(2):211-7
Hepatocyte membrane injury and bleb formation following low dose comfrey toxicity in rats.
This study attempted to prove the toxicity of low level doses of comfrey by using isolated Pyrrolizidine alkaloids extracted from the plant.
This study was not done on the safety of the entire plant, only certain isolated constituents. This study should not be used to determine the toxicity of the whole plant.
T. Furuya and K. Araki (1968) Studies on Constituents of Crude Drugs. (Alkaloids of Symphytum officinale Linn. Chem. Pharm. Bull. 16 1512-2516
Pharmacological tests with rats showing symphytine to have a LD50 of about 300mg/kg; that in an intravenous injection of 300 mg of the purified alkaloid per Kg of rat tissue caused death in approximately 50% of the experimental animals. Thus in the case of comfrey tea, it is to be assumed that normal methods of infusion could extract just over half the alkaloid that was extracted by 8 hours in a Soxhlet apparatus in the laboratory, each cup of tea could contain 100 micrograms of alkaloid.
At this level the consumer could never attain the lethal dose of 300 mg/Kg tissue found necessary to produce the acute reaction in rats. Even to consume this quantity it would take a 150 lb man drinking 4 cups of tea per day a total of 140 years. This scenario is unrealistic. Comfrey leaf is not toxic unless copious quantities are consumed for long periods of time.