Using metoprolol and diltiazem together may lead to increased side effects. This can cause fatigue, headache, fainting, swelling of the extremities, weight gain, shortness of breath, chest pain, decreased heartbeat/A – V block, or irregular heartbeat. If you take both medications together, tell your doctor if you have any of these symptoms. You may need a dose adjustment or need your blood pressure checked more frequently to safely use both medicines. It is important to tell your doctor about all other medications you use, including vitamins and herbs. Do not stop using any medications without first talking to your doctor. However if for any reason it becomes essential to use such combination, frequent monitoring of cardiac rhythm and cardiac contractility (by echocardiography) is advisable especially if baseline LV function is subnormal. Thanks