Natural product to modern discovery of their active ingredients and eventually to the development of new drugs. A large number of currently prescribed drugs have been either directly derived from or inspired by natural products. A few representative examples are listed below. Some of the oldest natural product based drugs are analgesics. The bark of the willow tree has been known from antiquity to have pain relieving properties. This is due to presence of the natural product salicin which in turn may be hydrolyzed into salicylic acid. A synthetic derivative acetylsalicylic acid better known as aspirin is a widely used pain reliever.
History of aspirin preparations became a standard part of the "materia medica" of Western medicine beginning at least with the Greek physician Hippocrates in the fifth century BCE; he recommended chewing on willow bark to relieve pain or fever, and drinking tea made from it to relieve pain during childbirth. The Roman encyclopedist Celsus, in his "De Medicina" of ca. 30 AD, suggested willow leaf extract to treat the four signs of inflammation: redness, heat, swelling and pain. Willow treatments also appeared in Dioscorides's "De Materia Medica", and Pliny the Elder's "Natural History". By the time of Galen, willow bark was commonly used