• an indole alkaloid • an α2-SELECTIVE ANTAGONIST • sometimes used in the treatment of orthostatic hypotension because it promotes norepinephrine release through blockade of presynaptic α2 receptors • once used to improve male erectile dysfunction but has been superseded by phosphodiesterase-5 inhibitors like sildenafil • can reverse the antihypertensive effects of an α2-adrenoceptor agonist such as clonidine