Elderberries are the dark purple fruit of the elderberry shrub. A rich source of antioxidants known as anthocyanins, elderberry is reputed by some to be effective in treating the common cold, flu, constipation, hay fever, and sinus infections. Others contend that it may be useful in treating toothache, sciatica, and burns, among other things, but some of these claims are better supported by research than others.