The herb comfrey contains potentially dangerous levels of toxic substances called pyrrolizidine alkaloids. For this reason, comfrey has been taken off the market as an oral supplement. However, topical creams containing comfrey extracts may be safe when used properly, and these remain available. Over the last three years, a series of double-blind studies have found evidence that topical comfrey is helpful for various conditions involving pain in the joints or muscles.
The most recent study enrolled 220 people with osteoarthritis of the knee. Over the three-week study period, participants applied either a placebo cream or a comfrey extract cream to the painful area three times daily. The results showed an improvement in mobility and pain in the comfrey cream group as compared to the placebo group.
Previous studies found benefits with comfrey cream for treatment of back pain and ankle sprain.
NOTE: Even with topical forms of comfrey, some concerns remain. See the full comfrey article for more information.