Alpha-adrenergic blocking agents, or alpha blockers, are medications that selectively block the effects of norepinephrine on alpha-adrenergic receptors in the body. They cause relaxation of smooth muscle in blood vessels, leading to decreased blood pressure and are used in the treatment of hypertension, benign prostatic hyperplasia, and urinary retention. Alpha blockers are also used to treat Raynaud's disease, pheochromocytoma, and erectile dysfunction. Examples of alpha blockers include doxazosin, prazosin, and terazosin.