There was a report out a year or so ago about a study done by the Army about medicine expiration dates (since they have a particular interest in how long they remain useful in storage.) Basically, their conclusion was that with a few specialized exceptions, pills don't expire. They have expiration dates because there are regulations that say they have to, but when they expire is basically a marketing ploy to get people to throw them away and buy more; it's not based on anything scientific. (Liquid medicines, on the other hand, apparently actually do lose potency or go bad.) Useful to know if you're trying to conserve medications.
Expiration dates