Hello, Welcome to Health Care Magic. Thanks for writing. I am Dr.Saddiq-ul-Abidin. I have read your question completely, I understand your concern and will try to help you in best way possible. I am sorry to know the symptoms you are having at the moment. Atrial fibrillation as you must be knowing causes increased beating rate of atria and this can eventually make ventricles to get increased rate so if we can t make atrial fibrillation to convert into normal rhythm we give some drug which at-least slows down the atrial contractions reducing the risk of embolus formation and ventricular rapid response. These drugs can be calcium channel blockers , beta blockers or other heart rate reduction drugs. What you have been taking is Verapamil which is a calcium channel blocker. It is not really a common side effect of calcium channel blocker to cause fatigue and dizziness , and specially when you have been tolerating verapamil from one brand , the other brand shouldn t be causing these symptoms, here You can deal with this matter in two ways. Consider other possibilities which can be causing these symptoms like lack of sleep , any other medicines, any use of sedatives, anti allergic, anxiety medicines, mental stress etc. If that is not there then check your intake of food salt and energy , if that is normal too, then meet your doctor he might want to check your hormonal levels like thyroid. Then You always have to chose another brand , there are many other brands in Verapamil, if this fails and you still have doubts that it is drug side effect you can chose other medicines used for atrial fibrillation like beta blockers, after discussing with your doctor. You just have to work with a strategy and every thing should be fine soon. I hope this answered your question.If you have more queries I am happy to answer.Otherwise rate and close the discussion Regrads. Dr.Saddiq ul Abidin M.B.B.S(Licensed Family Physician) Resident Medicine.