An alpha blocker is a medication that helps to relax and widen blood vessels. Alpha blockers, also called alpha-adrenergic agonists, are often used to treat high blood pressure, enlarged prostate, and other conditions in which blood flow becomes restricted. An alpha blocker blocks the action of a hormone, called norepinephrine, that prevents the tightening of muscles within small veins and arteries, which in turn results in relaxed, widened blood vessels. An alpha blocker promotes better blood flow and lower blood pressure. Alpha blockers can also help other muscles in the body relax, so they can also help ease the flow of urine in men with enlarged prostate.