I don't think of it like milk or driver's licenses :-) They might become less effective but also might not. I was watching a news segment where for drugs they put an "arbitrary" date on there so presumably you'll use them like you're supposed to or "need" to go to the doctor and get more, etc. but that they often don't really "go bad" or anything. I know with vitamins some are more/less sensitive to heat/light, etc. but I don't think they'll "suddenly" go bad.

I had a funny thing happen the other day though. I needed a battery for my smoke detector so found a new pack and changed it but they didn't work, it kept telling me the new batteries were dead. I finally looked on the packaging and they expired in 2004! I bet I got them at a dollar store and I bet I know why they were only $1.00 :-) I've been burned by the dollar store near me before (charged me 7 times for an item and I didn't discover it until I got home) so am not going there anymore.