Phenoxybenzamine !
An irreversible alpha blocker used before surgery for pheochromocytoma, to control the hypertension and sweating caused by the tumour's catecholamines.
Updated 2026-11-01
Mechanism of action
Phenoxybenzamine forms a reactive ethyleniminium intermediate that binds alpha receptors covalently, so the block cannot be overcome by more catecholamine and lasts until new receptors are made. This 'chemical sympathectomy' widens skin, mucosal and abdominal vessels and lowers blood pressure in pheochromocytoma.
- Phenoxybenzamine forms a reactive intermediate
- It binds alpha receptors covalently: the block is irreversible
- Catecholamines can no longer constrict vessels
- Blood pressure falls until new receptors are made
Pharmacokinetics
- Absorption
- Only about 20 to 30 percent of an oral dose is absorbed in active form (label)
- Half-life
- About 24 hours after intravenous use (label); effects persist for at least 3 to 4 days
Indications
- Pheochromocytoma: control of hypertension and sweating, usually before surgery Phaeochromocytoma →
Adverse effects
Common
- Postural hypotension and dizziness
- Tachycardia
- Nasal congestion
- Miosis
- Inhibition of ejaculation
Contraindications
- Conditions in which a fall in blood pressure is undesirable
- Hypersensitivity to phenoxybenzamine
Key interactions
- Epinephrine and other drugs that stimulate alpha and beta receptors
- Exaggerated hypotension and tachycardia, because beta effects are unopposed
- Other antihypertensives
- Additive hypotension
What is monitored
- Blood pressure lying and standing
- Heart rate
Nursing care
- Check blood pressure lying and standing; teach to rise slowly.
- Explain that nasal stuffiness and small pupils are expected effects.
- Report fainting or a very fast heartbeat.
- Caution in marked cerebral or coronary arteriosclerosis and kidney damage.
Pregnancy and breastfeeding
Human data are limited; in pheochromocytoma during pregnancy the benefit of controlling catecholamine crises is weighed against the risk.
Test yourself
Think of the answer, then tap the question to see it.
Why is the block produced by phenoxybenzamine irreversible?
It forms a reactive ethyleniminium intermediate that binds alpha receptors covalently, so recovery needs new receptors.
What is phenoxybenzamine mainly used for?
Controlling hypertension and sweating in pheochromocytoma, usually as preparation for surgery.
Why can epinephrine lower blood pressure in a patient on phenoxybenzamine?
With alpha receptors blocked, epinephrine acts on beta-2 receptors, which dilate vessels, causing hypotension and tachycardia.
Related drugs
Related conditions
← Back to the class: Alpha blockers
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - PHENOXYBENZAMINE HYDROCHLORIDE capsule (DailyMed (U.S. National Library of Medicine), 2026); Phenoxybenzamine: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2017); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Farmacología básica y clínica (Katzung), cap. 10 (McGraw Hill, 2025).
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