Lopressor^® (metoprolol tartrate) is a prescription medication that is used to treat several conditions related to the heart and blood vessels, including high blood pressure and angina. The drug can also be used to improve survival following a heart attack. Part of a class of drugs known as beta blockers, Lopressor can decrease heart rate, blood pressure levels, and the workload of the heart by causing the reverse effect of certain stress hormones in the body. Dizziness, tiredness, and a slow heart rate are a few common Lopressor side effects. Some of the conditions you should let your healthcare provider know about before you take Lopressor include hyperthyroidism, Raynaud's phenomenon or other circulation problems, and any allergies you may have.