Expiration dates are generally *very* conservative. My source Dr. Dean Udell. He read from a medical journal the other day about a study that showed only slight loss of potency after 5, 7, and, yes, even 10 years after the expiration date.
Botton line was the tested medication where still viable.
sk, on this I will have to disagree with you.
I would also think that environmental conditions during storage would also play a factor. UV and temp would be 2 critical factors to potency degradation.