Contemporary herbalists view comfrey as an ambivalent and controversial herb that may offer therapeutic benefits but can cause liver toxicity.

One of the most common uses of Comfrey extract is as a skin treatment. The plant contains the small organic molecule allantoin, which is thought to stimulate cell growth and repair while simultaneously depressing inflammation.^[4] Scientists and medical doctors agree that the use of Comfrey should be restricted to topical use, and should never be ingested, as it contains dangerous amounts of hepatotoxic pyrrolizidine alkaloids (PAs).^[5][6] Use of comfrey can, because of these PAs, lead to veno-occlusive disease (VOD). VOD can in turn lead to liver failure, and comfrey has been implicated in at least one death.^[7] In 2001, the United States Food and Drug Administration issued a warning against internal usage of herbal products containing comfrey, and eventually banned Comfrey products intended for internal use.^[8][9] In addition to restrictions on oral use, scientists and medical professionals recommend applying Comfrey extracts no longer than 10 days in a row, and no more than 4–6 weeks a year.^[4][10]

One of the country names for comfrey was ‘knitbone’, a reminder of its traditional use in healing bone fractures. Modern science confirms that comfrey can influence the course of bone ailments.^{[11][12][13][14]}

The allantoin contained in the plant is thought to help replace and thus repair cells in the body through its profliferant properties. Comfrey was used in an attempt to treat a wide variety of ailments ranging from bronchial problems, broken bones, sprains, arthritis, gastric and varicose ulcers, severe burns, acne and other skin conditions. It was reputed to have bone and teeth building properties in children, and have value in treating "many female disorders". Constituents of comfrey also include mucilage, steroidal saponins, tannins, pyrrolizidine alkaloids, inulin, and proteins.^{[citation needed]}

The flowers of Russian comfrey

Comfrey contains excessive doses of symphytine, one of the PAs in comfrey, may cause cancer in rats.^[15] This was shown by injection of the pure alkaloid. The whole plant has also been shown to induce precancerous changes in rats.^[16]

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.^[17]