http://goo.gl/59wHuV
Many drugs can alter vascular tone, or the degree of constriction in blood vessels. When the blood vessels tighten, blood pressure increases, and when it is too high it can lead to heart attack or stroke. Conversely, when blood pressure is too low people may experience lethargy, sexual dysfunction and other problems. Some drugs designed to affect vascular tone can also have cascading effects in the brain. Whether these unintended outcomes are helpful or harmful remains an open question.