Quote from here:
http://www.health.harvard.edu/fhg/updates/update1103a.shtml
"Most of what is known about drug expiration dates comes from a study conducted by the Food and Drug Administration at the request of the military. With a large and expensive stockpile of drugs, the military faced tossing out and replacing its drugs every few years. What they found from the study is 90% of more than 100 drugs, both prescription and over-the-counter, were perfectly good to use even 15 years after the expiration date. "
DO NOT take Tetracycline after the expiry date, that's a dangerous one, IIRC.
JCM