According to the Harvard Medical School Family Health Guide, it is safe to use a drug past its expiration date. This indicates the last date the manufacturer guarantees that a drug is at its full strength. Expired Tylenol, at worst, loses some of its potency.
The Family Health Guide describes an FDA study done in cooperation with the military. More than 100 prescription and over-the-counter medications were researched, and 90 percent of them were acceptable for use 15 years after their expiration dates. Drugs that do seem to expire include nitroglycerin, insulin, liquid antibiotics and, perhaps, tetracycline. To help medicines stay effective, they should be kept in cool, dry locations.