With curdled milk or petrified cucumbers it's easy to see when your food is past its prime; it's a little trickier with medicines. During the late 1970s, FDA began requiring an expiration date on prescription and over-the counter medicines. They are a critical part of determining whether the product is safe to use and will work as intended.
There is no assurance that drug products held past their expiration dates are safe or effective due to possible changes in chemical composition or a decrease in potency.
Improper storage–such as a humid bathroom cabinet–can also contribute to decreased effectiveness in medicines that have not reached their expiration date.