Prazosin is a quinazoline derivative, which lowers blood pressure by selective alpha-1-receptor antagonism. It has been found that, for similar decreases in blood pressure, prazosin causes less reflex tachycardia than non-selective alpha blockers or direct vasodilators, and that it is as efficacious as other conventional antihypertensive agents (alpha-methyldopa, hydralazine, beta blockers, etc.) in controlling severe hypertension. Prazosin causes a dose-related decrease in blood pressure in humans; twice-daily dosing is sufficient for 24-hour blood pressure control. Prazosin in combination with other drugs has also been shown to control blood pressure effectively, and it has been associated with relatively few side effects. Other agents with selective alpha-1-receptor-blocking activity are under investigation: these include doxazosin, trimazosin, terazosin, and alfuzosin.