A vasodilator is any pharmaceutical that has the effect of relaxing the smooth muscles of blood vessels, allowing blood to flow through them more easily. In the body and the environment, several chemicals do this naturally, such as histamines, adenosine, niacin, and even alcohol.
In medicine, vasodilators are used to treat high blood pressure, angina, and congestive heart failure. Viagra is also a vasodilator and is used to treat erectile dysfunction.