The active ingredient in all aspirin on the market as of 2014 is a synthetic compound called acetyl salicylic acid. This is a derivative of the compound salicin. Plants and trees, most notably the willow tree, naturally produce salicin.
Synthetic aspirin was first discovered by a German chemist named Felix Hoffman in the year 1897. For many years before this discovery, salicin, the naturally occurring form, was used to treat fevers and relieve pain. Because the natural salicin compound irritated the lining of the stomach and mouth, it rarely was prescribed for treating ailments. Hoffman remedied this problem when he discovered how to artificially synthesize salicin, and he began to commercially produce aspirin.
As early as the year 400 B.C., willow tree extract was used by the Greeks for medicinal purposes. Famous Greek philosopher Hippocrates prescribed a drink made with willow tree extract to help mothers with labor pains. In the year 1763, an English reverend named Edward Stone conducted the first scientific study on the extract. He gave ground willow tree bark to 50 people suffering from rheumatic fever, recorded the results of the experiment and proved that the bark helped to lower fevers and stop pain and inflammation.