Black cohosh (or blue cohos) is a perennial herb that is used by naturopaths for the symptoms of the menopause, premenstrual discomfort, painful periods and arthritis. Clinical trials show only limited effectiveness. Its common adverse effect is stomach discomfort, but it has been shown to cause significant liver damage and block electrical signals in the heart in some patients. It may interact with other medications including oestrogen. It should not be used during pregnancy (as it stimulates the uterus), while breastfeeding or in people with heart disease.