Metoprolol (which is sold under the brand names Lopressor, Toprol, Toprol XL) is a type of medication known as a beta blocker. This class of medications works by decreasing strain on the heart. Metoprolol is most commonly used to prevent, treat, or improve symptoms for high blood pressure, chest pain (angina),heart failure, fast, irregular heartbeat like atrial fibrillation or atrial flutter, and heart attack (myocardial infarction). Here’s more on what you need to know about metoprolol, its primary uses, and what side effects to be aware of before taking the drug.