Most of us have vitamins in our cabinets, and like drugs, and eggs, and milk, they have expiration dates. If the supplement has expired, often, the date on the bottle is required by law from the manufacturer since the potency of vitamins will degrade over time.
You can safely continue to consume most vitamin supplements past their expiration, however, their ability to work in your body will be diminished, and certain ones that require refrigeration, must be returned to the refrigerator soon after using them for maximum safety, potency and freshness.
The NY Times offers a very good explanation of vitamin expiration dates and when to dispose of older supplements.
Check it out here