For drugs the situation is different. If drugs are used within the expiration date, the manufacturer guarantees that they work as expected. After that date, they may still work, but they are no longer guaranteed. A study done by the U.S. Food and Drug Administration covered over 100 drugs, prescription and over-the-counter. About 90% of them were safe and effective as far as 15 years past their expiration date. Joel Davis, a former FDA expiration-date compliance chief, said that with a handful of exceptions - notably nitroglycerin, insulin and some liquid antibiotics - most expired drugs are probably effective.