Manufacturers generally mark expiration dates on aspirin bottles for two or three years after packaging, but studies have shown that aspirin retains 100 percent potency after four years or even longer. Storing aspirin in cool, dark locations such as a closet shelf keeps the aspirin potent longer.
Manufacturers are required to publish a shelf life that guarantees at least 90 percent potency of the drug. Many people store aspirin and other drugs in their bathrooms, an environment that is higher in humidity than the rest of the house. A humid environment saps many drugs of their potency, which often reduces shelf life significantly.