The bark of white willow (Salix alba) has been used for thousands of years as an analgesic. As many people know, willow bark contains salicin, which can be converted either synthetically or by the body to salicylic acid. This in turn has been converted synthetically to acetylsalicylic acid, which is called aspirin. Herbalist have found that extracts of white willow bark are able to combat similar pain, fever, and other symptoms that aspirin will also address. Extracts of willow bark now exist that are standardized to contain various amounts of salicin compound. These are useful to determine dosing of recurrent pain such as headaches.