My wife and I received a mail about a $500-something dollar epipen having reached its expiration date. It made me curious whether the "obsolescence" of the pharmaceuticals equated to actual risk.
I found a decent 2012 Science-Based-Medicine journal article by Scott Gavura, seeking answers to the question, and found once-again that medical ethics are rich in direction for environmental ethics. Human Health has been a concern for longer than Environmental Health.
So basically the article says that there is very little risk that expired medicine is bad for you. It doesn't turn into poison (there was one possible case of that from a medicine that was long ago banned from the market... think of the liability if people died from not reading the date on your label).
When a new medicine is approved by FDA, no Pharma company can afford to then test it by putting it on the shelf for several years to determine its expiration date. They do run tests on exposure to moisture and light, and use those to predict shelf life. But like food, an open can of stuff doesn't stay good for as long as a closed can of stuff, so the expiration date is majorly affected by whether it is pre-consumer (unopened at a pharmacy) or post-consumer (excess from a once opened bottle).
And this is hot topic in Waste Policy... see all the national pharma take-back day events this month.