 Salicin is also commonly found in the bark of "Populus" species, and the leaves of willows and poplars.It is also found in castoreum, which was used as an analgesic, anti-inflammatory, and antipyretic.The activity of castoreum has been credited to the accumulation of salicin from willow trees in the beaver's diet, which is transformed to salicylic acid and has an action very similar to that of aspirin.