First, let’s start by explaining what the expiration date actually is.
The expiration date is the last day an item will be at its highest level of potency. This means that the manufacturer can successfully guarantee the quality of a product up until the expiration date. A quality manufacturer will ensure that their vitamins are at the listed strength on the label at the time of expiration. If the manufacturer does not list an expiration date, then they are liable for the listed strength for the entire time the product is in circulation.
The U.S. Food and Drug Administration (FDA) does not require supplements to have expiration dates. However, quality manufacturers voluntarily list expiration dates because it is to their benefit. They test their vitamins rigorously to make sure they can successfully guarantee the label strength up until the expiration date.
So ultimately, just because a product reaches it's expiration date, that doesn’t mean it’s no longer any good; it only means that label strength may be lost over time.
Expiration dates are extremely conservative. All vitamins start to break down the day they are manufactured. Vitamins deteriorate at different rates, so manufacturers will beef up the strength of some vitamins that tend to deteriorate faster, in order to hit the listed strength at the time of expiration.
If you were to purchase vitamins on the day they’re manufactured, in some cases you’re actually getting a higher strength than what’s listed on the label (to allow for deterioration over time). Many expiration dates are also padded to factor in such things as shelf time and shipping time.
Vitamin manufacturers expect that their vitamins will spend some time traveling to their warehouse destination, then spend some time on stores shelves, before arriving in the customer’s hands. Vitamin strength and expiration dates are carefully coordinated to guarantee the listed label strength up until the day of expiration.