Some drugs will lose a bit of potency as they age. For the vast majority of drugs any of us are likely to take, who cares? If I pop a 325mg Tylenol into my mouth, but because it's ten years old it only functions at the 320mg level, is that a big deal?

This may not be the case for drugs that have very tight dosages for your particular condition. Something like digoxin is not a drug I would take after its expiration date, except under dire circumstances.

Chances are the reduced potency old pill will not hurt you any, it just may not help you as much as it would have at its full original potency.