"Take two aspirin and call me in the morning" has become the standard cliché for dealing with any minor illness.
Salicylic acid is the chemical in aspirin that eases high blood pressure, headaches, arthritis, and pre-menstrual syndrome. Use of salicylic acid dates back at least to the Ancient Greeks, who extracted the drug from certain trees and shrubs. But salicylic acid alone can cause serious stomach irritation.
How Does Aspirin Work?
A part of the solution to these side-effects was found in the mid-nineteenth century, but the significance of the discovery wasn't realized until about forty years later. In 1853, chemists discovered that salicylic acid would react with acetic acid to form yet a third acid, which they called "acetylsalicylic acid."
The long name isn't important, but this combination acid turned out to be the most common drug in the twentieth century.
Felix Hoffman's New Compound "Aspirin"
In the 1890's a young scientist, named Felix Hoffman, was looking for a cure for his father's terrible arthritis. Hoffman noticed that acetylsalicylic acid produced the benefits of the earlier drug without the side effects. In 1899, the new compound was first sold under the name of "aspirin."
Salicylic acid and acetic acid can both irritate the stomach, but when they are joined by a chemical reaction, the result is safe for most people.
Unfortunately, air, heat, and moisture, can reverse that reaction turning your aspirin back into salicylic acid and acetic acid, both of which are harmful to the stomach lining. Since acetic acid is a concentrated form of vinegar, you can smell a bottle of aspirin to tell if it's getting old. If aspirin smells strongly of vinegar, throw it away; it's no longer aspirin.