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 only to a lesser extent. A rare exception is a case of renal tubular damage said to have been caused by out of date tetracycline. A study done by the U.S. Food and Drug Administration covered over 100 drugs, prescription and over-the-counter. The results showed that 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.