I'm sure that there are folks who have used really old meds that seemed to work OK, but as far as scientific evidence goes, there is little. The Department of Defense's Shelf Life Extension Program demonstrated that most (but not all) samples they tested were still usefully potent several years past expiration. However, one major caveat is that I believe that they only evaluate unopened meds, which have been stored properly.

With an old, opened bottle of acetominophen that you keep in the medicine cabinet--I don't think anyone can say with much confidence without actually testing it since there is so much variation in temperature and humidity from one person's situation to the next.

So...not really a definitive answer, but that's pretty much about the extent of the scientific research. The rest will be personal accounts, which can be useful, but maybe only apply to their own situation.