Usually the time limits on drugs are based on the expectation that at the end of a certain length of time, the drug will have lost some % of efficiency, which would of course throw off the dose calculations that are based on a maximum effectiveness. So many drugs will still be useful even some years after their expiration if they have never been opened or exposed to heat or freezing. HOWEVER there are many that just break down with age, like amoxil suspension, into who knows what. Other suspensions like kaopectate separate but are still all right. If it changes color or texture or looks oily, toss it!