Alpha blockers are also called alpha-adrenergic antagonists. They treat a variety of conditions like high blood pressure, benign prostatic hyperplasia (BPH) and Raynaud's disease. These drugs relax the muscles and help small blood vessels to remain open. They work by preventing the hormone norepinephrine (noradrenaline) from contracting the muscles in the walls of smaller arteries and veins. Blocking that effect causes the vessels to remain open and relaxed. This improves blood flow and lower BP.