Not a good idea tbh...
http://en.wikipedia.org/wiki/Comfrey
Medicinal uses
Contemporary herbalists view comfrey as an ambivalent and controversial herb that may offer therapeutic benefits but can cause
liver toxicity.
Internal usage of comfrey should be avoided because it contains
hepatotoxic pyrrolizidine alkaloids (PAs). Use of comfrey can, because of these PAs, lead to
veno-occlusive disease (VOD). VOD can in turn lead to
liver failure, and comfrey, taken in extreme amounts, has been implicated in at least one death.
[6] In 2001, the
United States Food and Drug Administration issued a warning against internal usage of herbal products containing comfrey.
[7] There are ways to remove the pyrrolizidine alkaloids from comfrey, and some herbal product manufacturers have begun doing so (although the products will still be labelled “for external use only”).
Excessive doses of
symphytine, one of the PAs in comfrey, may cause
cancer in rats.
[8] This was shown by injection of the pure alkaloid. The whole plant has also been shown to induce precancerous changes in rats.
[9]
Most recently, in a placebo controlled study comfrey was found to decrease back pain when used topically. However, it is not clear if these results reached
statistical significance.
[10]
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