Phenoxybenzamine is a non-selective, non-competitive, long-acting α-blocker. It is often used for alpha blockade prior to phaeochromocytoma surgery.
Its non-competitive antagonism may reduce the effects of catecholamine surges, but may be implicated in postoperative refractory (catecholamine-resistant) hypotension
It should therefore be stopped 24–48 h before surgery due to its long half-life.
Its non-specific nature also allows pre-synaptic α2-blockade which interferes with the noradrenaline negative feedback loop that regulates noradrenaline release. The resulting uninhibited release of noradrenaline from cardiac sympathetic neurones causes a reflex tachycardia via β1 stimulation.
Central α2-blockade also results in somnolence, headache, and nasal congestion