However, unlike labeling for aspirin, labels for these products do not carry the FDA warning that children and teenagers should not use these products if they have symptoms of chickenpox or influenza, because such use has been associated with Reye syndrome, a rare, but serious illness. Levels of natural constituents in herbs sometimes vary greatly. The concentration of a constituent, for instance, varies with the part of the plant it comes from, growth stage at harvest, and processing and dilution during manufacture. Some ingredients in herbal products naturally contain chemicals that are harmful, such as certain alkaloids. Since 1985, at least seven cases of liver disease, with one death, have been associated with oral use of products made from this leafy plant.
Comfrey contains pyrrolizidine alkaloids.
People who ingest even small amounts of these alkaloids for a prolonged time may be at risk for developing cirrhosis of the liver. Comfrey is sold as teas, tablets, capsules, tinctures, medicinal poultices, and lotions. Use of this nonessential element for a long time may cause serious, irreversible kidney damage and has resulted in death. There is no evidence of effectiveness. One brand resulted in hospitalization of at least 10 patients and one death from a blood clot after surgery to remove a throat blockage.