Actually, expiration date is the date up until which the drug manufacturer can guarantee that the medicine is fully potent and safe to take based on product testing. Expiration dates are typically conservative to make sure you get what you paid for—a fully potent and safe medicine.

The effectiveness of a medicine may decrease over time, but studies have shown that much of the original potency still remains years after the expiration date. Excluding certain prescription medicines such as nitroglycerin, insulin, and liquid antibiotics, most medicines stored under reasonable conditions retain at least 70% to 80% of their original potency for at least 1 to 2 years after the expiration date, even after the container has been opened.1 So, if you have a headache one night, and you reach for your bottle of ibuprofen and find that it expired last year, chances are high that the medicine still has retained most of its potency.

However, it is still advisable to replace medicines that have expired years ago with a new supply to be sure you are using the most up-to-date product with the most up-to-date instructions for use. Since you last purchased the product, new dosing instructions or warnings may be advised; the strength may have changed to reduce the risk of errors; a new dosing device may be available to help measure doses more accurately; the product may be packaged in a new container more child-resistant than an older version of the drug; and so on. So, clean out your medicine supplies regularly, and replace any medicines that are more than a year or two beyond its expiration date.

Reference: 1) Drugs past their expiration date. The Medical Letter on Drugs and Therapeutics. October 8, 2002;44(1142):93-94.