The most concentrated part of the tree (bark) is used from white willow. This dates back as far as 5th Century BC where it was used to bring relief to a range of ailments! Most commonly used for ease of movement in horses and general well being. Horses seem to appreciate and instantly seek this natural ingredient in their paddock (such as willow trees). However, it is the bark that has the most potent level of active to provide relief. This active salicin in the bark is similar in structure to modern day medicine and what we recognize as Aspirin.