<unk> , sold under the brand name Lopressor, among others, is a selective β1 receptor blocker medication. It is used to treat high blood pressure, chest pain due to poor blood flow to the heart, and a number of conditions involving an abnormally fast heart rate. By working on the beta-1 receptor of the cardiac muscle cells, it yields both a chronotropic and inotropic effect. It is also used to prevent further heart problems after myocardial infarction and to prevent headaches in those with migraines.<unk>  is sold in formulations that can be taken by mouth or given intravenously. The medication is often taken twice a day. The extended-release formulation is taken once per day.<unk>  may be combined with hydrochlorothiazide (a diuretic) in a single tablet. Common side effects include trouble sleeping, feeling tired, feeling faint, and abdominal discomfort. Large doses may cause serious toxicity. Risk in pregnancy has not been ruled out. It appears to be safe in breastfeeding. The<unk> abolism of<unk>  can vary widely between patients, often as a result of hepatic impairment or CYP2D6 polymorphism. Care should be taken in patients with asthma;<unk>  should only be used in these patients when the benefits outweigh the risks, for example in heart failure. Stopping this drug should be done slowly to decrease the risk of further health problems.<unk>  was first made in 1969, patented in 1970, and approved for medical use in 1982. It is on the World Health Organization's List of Essential Medicines. It is available as a generic drug. In 2019, it was the fifth most commonly prescribed medication in the United States, with more than 74 million prescriptions.
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