Comfrey is used as a tea for upset stomach, ulcers, heavy menstrual periods, diarrhea, bloody urine, persistent cough, painful breathing (pleuritis), bronchitis, cancer, and chest pain (angina).It is also used as a gargle for gum disease and sore throat.roken or damaged skin: Don’t apply comfrey to broken or damaged skin. Doing so might expose you to large amounts of the chemicals in comfrey that can cause liver damage and other serious health effects. Liver disease: There is a concern that comfrey might make liver disease worse.