Willow bark has been used since ancient times in folk medicines around the world as a remedy for pain and fevers. In 1830, a French pharmacist called Leroux isolated the compound responsible for these effects from the bark. He named it 'salicin' after the botanical name of willows, Salix. Research continued with Raffaele Piria who made salicylic acid out of salicin in 1838. All this chemical work led to the invention of aspirin (also known as acetylsalicylic acid) at the end of the century.

Salicylic acid is listed by the World Health Organization as a ‘drug affecting skin differentiation and proliferation’. In fact, the material is a keratolytic agent, which means that it softens the outer layer of the skin (epidermis) and causes desquamation - an effect that could be compared to a powerful exfoliation. It may sound dangerous, but used with care and in the right amount, the acid is safe and very helpful in treating acne, dandruff, and many skin conditions such as psoriasis and seborrheic dermatitis.