With a few specific exceptions, it is generally safe to use drugs that have passed their expiration dates. We know this thanks to a study carried out some years ago by the FDA at the request of the Department of Defense, which maintains enormous stores of drugs of all kinds for use by the military. Defense authorities wanted to know whether the drugs in its warehouses were potent and worth keeping after their expiration dates had passed. The study showed that 90 percent of more than 100 drugs, both prescription and over-the-counter, were perfectly good to use even 15 years after the expiration dates.
Since 1979, the FDA has required drug manufacturers to test their products to see how long they last and then affix an expiration date to the packaging. Manufacturers don’t keep testing their products indefinitely to see when they reach a point where they no longer work. Instead, they perform accelerated testing in harsh conditions of heat and moisture or use normal condition testing for a year or two, and if a product remains effective after the time is up, they stamp a corresponding expiration date on it. Based on what the FDA learned when it investigated military drug supplies, the pills you take should retain their potency years beyond those dates. That goes for both prescription and non-prescription drugs.
The exceptions I mentioned above include the antibiotic tetracycline, which should never be taken past the expiration date because of a danger of kidney damage. Other drugs that degrade quickly are nitroglycerin, insulin, and liquid antibiotics. In addition, EpiPens used as emergency treatment for severe allergic reactions lose potency past their expiration dates.
You can help prolong the efficacy of your medications (and dietary supplements) by storing them in a cool, dry place. Don’t keep them in the medicine cabinet in the bathroom or in the kitchen where they will be exposed to heat on a regular basis - the bathroom gets hot and steamy with each shower, and the kitchen can overheat whenever you cook or bake.
The same general rules on proper storage apply to any products bearing expiration or "sell by" dates. For example, eggs graded by the USDA usually are okay to use four or five weeks after the sell-by date stamped on the carton as long as they're kept refrigerated. After that, eggs begin to dry out.
Be sure to check the expiration date on your sunscreen, too - if there isn’t any, the U.S. Centers for Disease Control and Prevention advises that you can assume the product is good for three years, providing that it hasn’t been exposed to high temperatures.
Here is a general rule to follow when considering whether to take pills past their expiration date: if the drug is for a serious medical problem, it’s best to get a fresh supply.
Andrew Weil, M.D.