About Phentolamine Mesylate A reversible nonselective alpha-adrenergic antagonist, Imidazoline derivative, Peripheral vasodilator ,Antihypertensive. Mechanism of Action of Phentolamine Mesylate Phentolamine is a quick & short acting non selective, competitive alpha adrenergic blocker and suppresses the presser actions of circulating catecholamines. It also blocks 5HT-receptors and potassium channels & causes release of histamine from mast cells. It produces dilatation of peripheral blood vessels, decrease peripheral resistance & blood pressure. It causes dilatation and relaxation of blood vessels and improves blood flow. When injected in to penis (intra-cavernosal) it increases blood flow to penis which results in erection.