Hello. Thank you for your question and welcome to HCM. I understand your concern. Every single drug has side effects, just like every procedure in medicine. But, they are only confined within a percentage of probability. Metoprolol succinate, taken 25 mg a day, is really the lowest dose we can prescribe. Atrial fibrillation is a benign supraventricular arrhythmia, if properly rate-controlled and anti-coagulated. It is not specified is this a paroxysmal atrial fibrillation (in which case metoprolol was prescribed to control the rate and to prevent episodes of A-Fib) or this is a chronic atrial fibrillation (in which case metoprolol was prescribed to control the rate and prevent rapid conduction to the ventricles). Whichever the case, I would also recommend you this kind of treatment. However, there changes in treatment can be mode and any drug that controls the conduction pathway via the atrio-ventricular node, by slowing it down, can be used. For this aim, we can use beta-blockers (atenolol, metoprolol), calcium channel blockers (verapamil, diltiazem), digitalis (digoxin, digitoxin), adenosine (which is reserved only for acute cases) and various antiarrhythmics (propranolol, amiodarone, flecainide). If the drug you are using is really worrisome to you, then you can discuss with your cardiologist about the above listed alternatives. I hope this helps. Regards, Dr. Meriton