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.
"I stretch lame hands of faith, and grope,
And gather dust and chaff, and call
To what I feel is Lord of all,
And faintly trust the larger hope.Ē
-Tennyson, In Memoriam