Blackberry root bark (Rubus macropetalus) is a good herbal remedy for diarrhea. It contains tannins, which have a desirable astringent action on the intestinal lining. Boil the root bark in water for twenty minutes, strain, and drink a cup every two to four hours till the diarrhea ends. You may be able to find tinctures of this useful plant in herb stores, or you can make your own if you live in an area where blackberries grow. Take a teaspoon of tincture in water every two to four hours.