Pharmacology Action: White Willow Bark contains salicin, which the body converts to salicylic acid and has the same effect on the body as aspirin without any of the side effects. In fact, White Willow Bark was the basis for the synthesis of aspirin. History of usage of White Willow Bark goes back as far as 500 BC when ancient Chinese healers began using it to control pain. Native Americans also discovered the value of the Willow tree for relieving pain from headaches and rheumatism and reducing fevers.