Answer:
It depends on how long after the expiration date they are used. While you will not get sick from it, i.e. it's not toxic, it may have lost its active properties with time. I personally, as well as many others I know personally, take medications past the expiration date with no adverse reactions. Again, really the only thing that can happen is that it may not be as powerful as it was if a long period of time has past since the expiration date.
I'm a pharmacy student and what I can tell you in general is that the expiration date is the date after which 10% of the active ingredient will decompose or otherwise breakdown. Basically the expired drug will have some loss of activity, but in most cases it shouldn't be too bad.