Willow bark is the historical source of salicylic acid, a precursor to aspirin. The connection begins with salicin, a natural compound found in several willow species, especially white willow (Salix alba). The body converts salicin into salicylic acid, which has pain-relieving and fever-reducing effects.
In 1828, Johann Buchner isolated relatively pure salicin from willow bark. Pierre-Joseph Leroux improved its extraction in 1829, and Raffaele Piria produced and named salicylic acid in 1838. Chemists later modified salicylic acid into acetylsalicylic acid, or aspirin, partly to reduce its irritating effects on the stomach.
A common mix-up is saying that willow bark contains aspirin itself. It does not: aspirin is a later, manufactured compound. Another nuance is that salicylic acid was also identified in meadowsweet, a plant in the genus Spiraea, but willow is the classic historical source associated with the aspirin story. Willow extracts are therefore related to aspirin historically, but they are not identical in dose, composition, or medical safety.