The US Food and Drug Administration on Friday asked makers of dietary supplements containing the herb comfrey to withdraw their products due to the danger of liver damage and its possible role as a cancer-causing agent. In a letter to industry groups representing herbal product makers, the FDA said it was aware of dietary supplements derived from three types of comfrey that contain toxic alkaloids. "The agency strongly recommends that firms marketing a product containing comfrey, or another source of pyrrolizidine alkaloids, remove the product from the market and alert its customers to immediately stop using the product,"FDA said. In addition to being toxic to the liver, 'there is also evidence that implicates these substances as carcinogens," the agency said. Separately the Federal Trade Commission (FTC) announced action Friday against an Internet marketer of comfrey remedies. Utah-based Christopher Enterprises Inc., agreed to a preliminary injunction that bars the company from marketing products containing comfrey "for internal uses or on open wounds," the commission said. The commission said the company falsely claimed it was safe to take comfrey products orally, as suppositories or to apply them to open wounds.