Inderal (Propranolol), is a beta blocking agent, the very first one developed. It is used to treat high blood pressure, anxiety , and panic. It is not an anti-depressant, and does not work the same way that anti-depressants do. Inderal blocks the action of epinephrine and norepinephrine, and these actions are immediate. The reason your doctor prescribed that you take it every 12 hours, is that it loses its effectiveness after that period of time. Feeling tired/lethargic is one of the main side effects, but this become less over time, as the body adjusts to the medication.
I also take a beta blocking agent, Lopressor, that I take twice a day, every 12 hours. I have been on it for 4-5 years now.
I hope this helps with your questions. The very best to you, Lexie!... Chuck