Diltiazem hydrochloride, a calcium channel blocker that is available by prescription, is commonly used to control high blood pressure and treat certain types of chest pain. The medication works by relaxing the blood vessels to help increase blood flow and make it easier for the heart to pump blood. Diltiazem is not suitable for everyone. A few of the conditions you should let your healthcare provider know about before taking the medication include heart disease, kidney disease, or any allergies you may have.