The most commonly used medications to control heart rate are metoprolol (MET), a beta blocker, or diltiazem (DT), a calcium channel blocker. Beta blockers are medications that cause the heart to beat more slowly and with less force. DT also helps blood vessels open up to improve blood flow. Both DT and MET are used alone or together with other medicines to treat severe chest pain (angina), high blood pressure (hypertension) or rapid heartbeat. Both are equally acceptable according to recent guidelines for AF/FL. There are limited studies comparing MET to DT for rate control for AF/FL.