Willow bark, the bark of several varieties of willow tree, has been used for centuries as a pain reliever. The active ingredient in the medicine made from willow bark is called salicin. Some people use willow bark as an alternative to aspirin, particularly those that experience chronic headaches or back pain. Willow bark is also used in some products to aid weight loss. It comes from the branches of 2- to 3-year-old willow trees. Willow trees and shrubs grow all over the world, except for Australia and Antarctica.