A vasoconstrictor is a substance or medication that causes blood vessels to narrow or constrict. This reduction in the size of blood vessels leads to a decrease in blood flow to various parts of the body and an increase in blood pressure. Vasoconstrictors are used to treat a range of medical conditions, including nasal congestion, headaches, and low blood pressure. Examples of vasoconstrictors include epinephrine (adrenaline), phenylephrine, and pseudoephedrine.