It's a beta blocker most commonly used to treat irregular/fast heart rates, but also used for anxiety. According to my pharmacology textbook, propranolol can cause difficulty breathing if overdosed for a healthy individual.
Why was it prescribed to you, for the reasons why could help explain if you could encounter any negative side effects.
Misplace one note, and there would be diminishment;misplace one phrase, and the structure would fall. It was clear to me that sound I'd heard in the archbishops palace had been no accident. Here again was the very voice of God. I was staring through the cages of those meticulous ink strokes -
...at an absolute beauty!...
- Antonio Salieri, Amadeus