"The date placed on the container/labels of a drug product designating the time during which a batch of the product is expected to remain within the approved shelf life specifications if stored under defined conditions, and after which it may not be used" (AMA, 2008).
Yeah … so what does all that mean? In everyday terms, a medication expiration date is the point at which a batch of drugs has reached the end of the longest period of time the manufacturer has tested the continued potency and safety of a medication. This process is known as stability testing.
There are a couple of things to note here. First, the manufacturer is only required to provide testing results to the FDA for the duration at which it tested a drug; it is not required to test a drug until it is no longer viable. Say the company tests a drug’s stability at two years on the shelf and determines that the drug has no changes at this point. Then the FDA will require the company to stamp the packaging with an expiration date two years from the date of manufacture. Is the drug still good after that date? Well, it wasn’t tested -- so no one can say for sure. (See When Do Medications Actually Expire? for a discussion on this topic.)
Another noteworthy blurb in the FDA definition is "stored under defined conditions." This little phrase means that if any condition is different from that which was tested -- storage, lighting, temperature, etc. -- the expiration date has been compromised. It is this that allows for beyond-use dating. (See Are Beyond-Use Dates Different Than Expiration Dates?)
American Medical Association. (2008, February). Report 1 of the Council on Scientific Affairs (A-01): Pharmaceutical expiration dates.