St John's wort. View of a St John's wort plant (Hypericum perforatum), which is used in herbal medicine. St John's wort grows wild throughout Europe in dry and sunny positions. The leaf tips and flowers are used to treat a variety of ailments, but are most commonly used as an antidepressant. Tablets made up of concentrated parts of the plant can be bought for this purpose. In traditional herbal medicine, the preparations were used for treating intestinal parasites, lung disorders, bladder problems and diarrhoea. In ancient times, the plant was used to ward off evil spirits.