There was a was Wall Street Journal article I read a few years ago about how the US armed forces has, in an effort to cut costs, established a program to determine how long drugs last post expiry date. What they found was that the vast majority were fine. Though the article also mentioned something i found interesting, that there were some drugs which the military made a conscious decisions not to use post expiry date due to possible PR ramifications. the main one being saline and ringers, that it would look absolutely terrible for an injured solider/sailor to look up in their bed to see that the NS drip running into their arm expired three years prior.
http://online.wsj.com/article/SB954201508530067326.html