What can happen if you take expired prescribed medication?

Nothing: Most medications may still be effective up to some time like 6 months after expiration.

Effects can vary: With rare exceptions (tetracycline, nitoglycerine and Insulin for example) most medications are generally safe upto 15 years after the expiration date. This was demonstrated by a study done by government on medications stored by the military. The date is required by the FDA to certify that a medication is fully potent. When in doubt avoid taking the medication