The Food and Drug Administration requires drug manufacturers to list an expiration date on all products. This date is determined after extensive testing for potency and stability.
Taking any medication after the expiration date listed by the manufacturer is never recommended mainly because the potency of the medication cannot be guaranteed. Most drugs lose potency after the expiration date to varying degrees but they usually are not harmful or toxic. However, depending on the medication and the condition for which it is taken, a small loss in potency can be potentially dangerous.
When you receive a medication dispensed by a pharmacy it is normally recommended that you do not use it after 6 months to 1 year from the dispensed date or the date indicated by the pharmacy. The stability of medications listed by a manufacturer is based on the medication stored in the original container and under controlled conditions.
Once you remove the medication from the original container and open the bottle you are changing those conditions.
As with any medication, make sure you ask your pharmacist or physician any questions you may have.