“Prescription medicine?”

Metoprolol comes in two forms: Metoprolol tartrate and metoprolol succinate. Both can be used for atrial fibrillation and hypertension. Although 300 mg of either is on the higher end of the dosing schedule, if it is working to keep your atrial fibrillation and blood pressure under control, then there is no need to be concerned. If your blood pressure gets too low or your pulse gets too low, you may need to decrease your dosage after talking to your doctor.  

300 mg of metoprolol (either long acting or immediate release) is above the usual recommended dose for that medication. Your doctor may have some specific reasons for using a dose that high, and it can be used in higher doses if the proper monitoring is done to ensure the heart rhythm is not altered at this dose. I would not increase the dose over 200 mg without specific guidance and follow up from your primary care provider or cardiologist.

Hello,

A typical high dose is 200mg daily. However, cardiologists sometimes prescribe off-label. Please ask a heart doctor to be sure.
Thank you!