Lopressor^® (metoprolol tartrate) is a prescription medication that is used to treat high blood pressure and angina symptoms. Taking the medication after a heart attack can also help increase the chances of survival. Lopressor works by reversing the effect of certain stress hormones in the body, which reduces heart rate, both diastolic and systolic blood pressure, and the workload of the heart. Lopressor comes in a tablet form and is available in two strengths. Lopressor side effects can include shortness of breath, itching or an unexplained rash, and depression. A few of the factors that can affect the Lopressor dosage your healthcare provider prescribes include how you respond to Lopressor, other medications you may be taking, and other medical conditions you may have.