This medication is used to treat high blood pressure (hypertension). Lowering high blood pressure helps prevent strokes, heart attacks, and kidney problems. This product contains 2 medications: atenolol and chlorthalidone. Atenolol belongs to a class of drugs known as beta blockers. It works by blocking the action of certain natural chemicals in your body, such as epinephrine, on the heart and blood vessels. This effect lowers the heart rate, blood pressure, and strain on the heart. Chlorthalidone is a "water pill" (diuretic) and causes your body to get rid of extra salt and water. This effect may increase the amount of urine you make when you first start the medication. It also helps to relax the blood vessels so that blood can flow more easily.