The substance in aspirin, acetylsalicylic acid, was first isolated from the bark of the willow tree (Salix species).

While not the exact same compound, the ancient Egyptians, Greeks, and Native Americans all used willow bark to treat pain and fever. This was due to the presence of salicin, a compound that the body converts into salicylic acid.

Later, scientists extracted and synthesized salicylic acid, which was found to be effective but had unpleasant side effects.

In 1897, German chemist Felix Hoffmann synthesized acetylsalicylic acid, which was more effective and had fewer side effects. This became the basis for the modern aspirin we know today.