You may have noticed that some supplements have expiration dates (also called “use by” or “best by” dates) while others don’t. That’s because, unlike prescription and over-the-counter meds, companies aren’t legally required to put expiration dates on supplement labels. However, companies can choose to print expiration dates on their products. If they choose to do so, they're required to have data proving the products will contain all of the ingredients—in the amounts listed—until the date on the label.
Most ingredients do break down over time, which can make supplements less potent; however, companies often include slightly higher amounts of ingredients than what's on the label to ensure the label is accurate by the expiration date. Storing supplements away from heat, humidity, and light, may help maintain their potency for about two years after the date of manufacture. Other products, like probiotics, may only last on the shelf for a year and may need to be refrigerated. Check labels for special storage instructions.
Source: New York Times