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simple, common comfrey has 5 separate analgesic chemicals! ascorbic acid, caffeic-acid, chlorogenic acid, selenium and thiamin. Plus the healing properties of allantoin, plus the anti-inflammitory properties of rosmarinic acid
It also has 8 chemicals with antihepatotoxic activity. So how does comfrey cause hepatotoxicity again? Oh, that's right... the roots contain pyrrolizidine alkaloids, and sometimes the leaves do too -although in significantly lower concentrations. Good thing I only use the leaves! To read the whole list, go here.

I worry about the potential hepatotoxic effects of pyrrolizidine alkaloids, so I follow the latest research very carefully. And the lastest research has me stumped. As an example: One dot gov site talks about the 4 deaths from liver failure that have been connected to comfrey usage. The most drastic case drank a few cups of comfrey tea and started having liver problems that eventually led to her death. Yet I find other studies (lots of them) where subjects regularly ingest comfrey and have no problems at all. As another example, the specific pyrrolizidine alkaloids in comfrey were injected into rats, and then the amount of pyrrolizidine alkaloids that was released in their urine and fecal matter was measured. And guess what? Nearly all of the alkaloids were eliminated within 14 hours. Also, comfrey root has up to 20 times more pyrrolizidine alkaloids than the leaves, and about 80% of the time, common comfrey leaves have no pyrrolizidine alkaloids at all (those that do seem to have cross-bred with other comfrey varieties). When used as a topical solution, less than half the alkaloids pentrated the skin. Also, many studies are unclear about which version of comfrey they used for testing. The reports I've found that specify Symphytum Officinale (common comfrey) show no side effects and results that clearly work. Comfrey relieves pain. Comfrey increases mobility. Comfrey speeds healing. I mean there are (finally) actual, scientific, double-blind studies showing the efficacy of comfrey as a topical application. Isn't that great?