A vasodilator is a substance or medication that causes the blood vessels to widen or dilate. This increase in the size of blood vessels leads to a decrease in blood pressure and an increase in blood flow to the body. Vasodilators are used to treat a range of medical conditions, including heart failure, angina, and high blood pressure. Examples of vasodilators include nitrates, such as nitroglycerin, and calcium channel blockers, such as diltiazem.