Best not to use it after the experiation date as it can run out. If you don't want to take it ask your psychiatrist specifically why it was prescribed and what they think is wrong that needs treatment.
I work in the medical field and this is what I know about medicine.
For pills and tablets the expirations date does indicate the date that the medicine will not work at full effectivness, which can have a negative effect on you. In general, pills don't "go bad" in that they will be rotten but gel tablets or other capsules may actually deteriorate in a negative way depending on the medicine. All medicines are different and you should not take expired pills, tablets, or casules for safety sake.
For liquid medicine you should never, ever risk taking it after the expiration date. Not only will it be less effective, but it can also begin to grow bacteria which can infect the lining of your digestive tract. This can be very dangerous.
I know it seems that medicines have expiration dates just to make you spend more money, but that is actually not the case. If you think you will not use your prescription for a year, it is best not to fill it until you need it. Talk with your doctor about this. A lot of times us bipolar people feel fine but we're actually not doing well, but we can't see it.
I hope everything goes well for you.