Elderberry is one of the most commonly used medicinal plants in the world. Traditionally, Native Americans used it to treat infections, while the ancient Egyptians used it to improve their complexions and heal burns. It’s still gathered and used in folk medicine across many parts of Europe. Today, elderberry is most often taken as a supplement to treat cold and flu symptoms. However, the raw berries, bark and leaves of the plant are also known to be poisonous and cause stomach problems.