List of products in the National Drug Code with proprietary name . This medication is used to treat or prevent many types of irregular heartbeats (heart arrhythmias such as atrial fibrillation). Quinidine can greatly improve your ability to perform normal activities by decreasing the number of irregular heartbeats you have. However, it may not stop all your irregular heartbeats completely. It works by blocking abnormal heartbeat signals. Before and while you are using quinidine, your doctor may prescribe other medications (e.g., "blood thinners"/anticoagulants such as warfarin, beta blockers such as metoprolol) to shrink any blood clots in the heart and to slow your pulse.