Recently published review of potency expired meds,28 to 40 YEARS old, suggests that many are still usable.

http://archinte.jamanetwork.com/article.aspx?articleid=1377417

unfortunately you need you need to subscribe to see the full article.

Good review of article here:
http://www.reuters.com/article/2012/10/08/us-expired-drugs-idUSBRE89712R20121008

and

http://newsfeedresearcher.com/data/articles_m41_3/drug-cantrell-medication.html