I wouldn't even assume that it is not as potent. In order for a company to put an expir. date on a drug, they have to do test for that long (REAL TIME). Sometimes they just want to go to market with the product so they don't run the expir. date experiments out for that long.
The shit might be perfect for years after after the date...they just don't have proof for it for the FDA. Think about it...what do the drug company care about long expir dates? Shorter ones mean more money for them.