Diltiazem is used to treat high blood pressure (hypertension) and prevent chest pain (angina). Lowering high blood pressure helps prevent strokes, heart attacks, and kidney problems. When used regularly, diltiazem can decrease the number and severity of episodes of chest pain from angina. It may help increase your ability to exercise. Diltiazem is called a calcium channel blocker. It works by relaxing blood vessels in the body and heart so blood can flow more easily. Diltiazem also lowers your heart rate. These effects help the heart work less hard and lower blood pressure.