Bottom Line: Comfrey has not been shown to treat or prevent cancer. Several cases of liver toxicity have been reported with its use.
Comfrey leaves and roots have been used for many centuries to treat several ailments. Comfrey contains two substances that have been found to be biologically active in laboratory tests: allantoin causes cells to increase the rate at which they divide, and rosmarinic acid reduces inflammation and prevents injury to the small blood vessels in the lungs. However, these effects have not been confirmed in humans through clinical trials.
Many cases of liver toxicity have been reported with use of comfrey.
To treat bronchitis
There is no scientific evidence to support this claim.
To treat cancer
This claim is not backed by research.
To treat peptic ulcers
There is no scientific evidence to support this.
To improve wound healing
Laboratory studies show that comfrey leaves have wound-healing effects. Human data are lacking.
Comfrey contains compounds that are toxic to the liver and can cause liver cancer.
Comfrey is a fast-growing plant whose leaves and roots have been used for centuries to treat many ailments, especially for wound healing.
A study done in mice indicated that oral comfrey has antiproliferative effects (13); the leaves showed wound healing properties (16); and the root extract was useful in relieving acute back pain (14)(17) and osteoarthritis of the knee (18). However, comfrey contains pyrrolizidine alkaloids that are hepatotoxic and carcinogenic (13).
There is extensive literature concerning hepatotoxicity with use of comfrey (5)(6)(7)(8)(9)(10). However, the risk of systemic absorption following the use of topical comfrey preparations is not known.
Comfrey was used since the 1930s for animal feed, but has been prohibited in Australia and New Zealand. In June 2001, the FDA asked all manufacturers to remove products containing comfrey from the market (12).
Comfrey herbal tea use resulted in several cases of cardiac glycoside poisoning due to misidentification with foxglove as the leaves of both plants look similar (15).
Triterpenes: Sitosterol and stigmasterol (phytosterols), steroidal saponins, and isobauerenol
Other constituents: Allantoin, caffeic acid, carotene, chlorogenic acid, choline, lithospermic acid, rosmarinic acid, and silicic acid (19)
Mechanism of Action
The mechanism of action has not been elucidated, but the therapeutic properties of comfrey are thought to be based on its anti-inflammatory and analgesic properties.
Comfrey was shown to stimulate granulation and tissue regeneration, and support callus formation (18). Allantoin and rosmarinic acid, two important components, are believed responsible for cell proliferation and anti-inflammatory effects (20).