Terazosin belongs to a class of drugs called alpha blockers, which relax certain muscles and help small blood vessels remain open. Alpha blockers work by keeping the hormone norepinephrine (noradrenaline) from tightening the muscles in the walls of smaller arteries and veins, which causes the vessels to remain open and relaxed. This improves blood flow and lowers blood pressure. Because alpha blockers also relax other muscles throughout the body, these medications can help improve urine flow in older men with prostate problems.