Generally a prescription is out of date if it is over a
year old. If it cam in an original manufactures package, you can go by the date on that.
I wouldn't risk using this kind of medication after expiration. Medications either lose their ability to work or become stronger. In this case, you really need it to work. If it has become stronger, the cardiac side effects could be too much for you to handle. I think this would be a dangerous drug to use out of date.