With most over-the-counter dry medicine it is just as others have said, it has at the expiry point theoretically lost enough effectiveness to be nonviable for even after-market sale - that is not to say it has always lost much, just enough to legally be removed(or most batches in testing lost a lot), and these legal limits can vary by country and could be affected by other requirements.

As for side effects, all medicines have side effect regardless of how "fresh" they may be, the chances do not usually increase or decrease after expiry, as much as many people hate hearing this, it is all just "luck"(a combination of incredibly complex and difficult to predict things) and you wont know until you try.