Expiration dates ensure products provide consistent results. While vitamins generally don't deteriorate into something harmful or toxic over time (if stored properly), it's possible for the potency to diminish. The FDA requires manufacturers that list an expiration date to maintain records proving that the product contains what the label states through date of expiration. To set expiration dates, the manufacturer must perform stability tests to determine active ingredient degradation over time, and should put enough of the raw material to compensate for any potency loss. A 2-year expiration date means the company certifies the product will continue to meet or exceed label claim for 2 years from date of manufacture. To get desired results, it's best not to take a supplement after its expiration date.