**Mechanism** - competitive non-selective alpha1 and alpha2 blocker - short duration of action - causes vasodilation and fall in BP - blocks effects of adrenaline and noradrenaline at alpha1 and alpha2 receptors - can result in reflex tachycardia with norad **pharmacokinetics** - IV - onset 1-2 min - duration 10-30 min **Indications** - phaemochromocytoma causing hypertensive crisis - inadvertent sympathomimetic extravasation - second line coronary vasodilating tx for cocaine associated myocardial infarction (after GTN or nitroprusside) **Contraindications** - hypotension - sulphite hypersensitivity - conditions where sudden decrease in BP is undesirable (eg stroke, CAD, heart failure → worsens reflex tachycardia) **Dose** - 1mg IV Q5 min up to 15mg max dose *for extravasation ischaemia:* - infiltrate area with 10-15mL of saline containing 5-10mg of phentolamine - use syringe with fine hypodermic needle, and infiltrate liberally throughout area - can have benefit within 12 hours