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Pharmacology · Alpha blockers

Phentolamine !

A short-acting, reversible alpha blocker given by injection: the antidote when norepinephrine leaks from a vein, and a control for pheochromocytoma crises.

Updated 2026-11-01

  • Class: Alpha blockers
  • Receptor: Alpha-1 and alpha-2 adrenergic receptors
  • ATC code: C04AB01

No structure of phentolamine bound to an alpha receptor is available; the figure shows doxazosin, another alpha blocker, in the binding site of the human alpha-1A adrenergic receptor.

Target in 3D: Alpha-1 and alpha-2 adrenergic receptors. PDB structure 9M4Q (RCSB PDB, public domain). View the structure at RCSB PDB

Mechanism of action

Phentolamine competitively and reversibly blocks alpha-1 and alpha-2 receptors. Vessels dilate and blood pressure falls; blocking presynaptic alpha-2 receptors increases norepinephrine release, which, with the baroreflex, causes marked tachycardia. Injected into tissue where a vasopressor has leaked, it reverses the local vasoconstriction.

  1. Phentolamine competitively blocks alpha-1 and alpha-2 receptors
  2. Vessels dilate and blood pressure falls
  3. Alpha-2 block increases norepinephrine release: tachycardia
  4. Local injection reverses vasopressor-induced ischaemia

Pharmacokinetics

Half-life
About 19 minutes after intravenous injection (label)
Onset and duration
Rapid after intravenous or local injection

Indications

  • Prevention or control of hypertensive episodes in pheochromocytoma, for example during surgery Phaeochromocytoma →
  • Prevention or treatment of skin necrosis after norepinephrine extravasation
  • Phentolamine test for pheochromocytoma (now largely replaced by biochemical tests) Phaeochromocytoma →

Adverse effects

Common

  • Hypotension and orthostatic hypotension
  • Tachycardia and arrhythmias
  • Weakness, dizziness and flushing
  • Nasal stuffiness
  • Nausea, vomiting and diarrhoea

Contraindications

  • Myocardial infarction, past or present
  • Coronary insufficiency, angina or other coronary artery disease
  • Hypersensitivity to phentolamine

Key interactions

Other antihypertensives and vasodilators
Additive hypotension
Epinephrine
The pressor effect is weakened and can reverse to hypotension

What is monitored

  • Blood pressure and heart rate closely after systemic use
  • ECG in people with heart disease
  • Skin colour and perfusion of the area treated for extravasation

Administration and safety

  • For extravasation, it is infiltrated through a fine needle into the blanched, ischaemic area as early as possible; it works best within the first hours.
  • Reconstitute the powder only with the diluent the product specifies and use it promptly.
  • Effective local treatment shows as immediate warming and reddening of the blanched skin.

Nursing care

  • Have phentolamine available wherever vasopressor infusions run.
  • After systemic use, check blood pressure and heart rate often; keep the patient lying down, legs raised if pressure falls.
  • Report chest pain, palpitations or neurological symptoms.
  • After local use, mark and recheck the area for colour, warmth and capillary refill.

Pregnancy and breastfeeding

Human data are limited; it is used if the benefit justifies the risk, for example in a pheochromocytoma crisis.

Test yourself

Think of the answer, then tap the question to see it.

What is phentolamine's role when a norepinephrine infusion extravasates?

It is infiltrated into the area to block alpha receptors and reverse the local vasoconstriction, preventing necrosis and sloughing.

Why does phentolamine cause more tachycardia than prazosin?

It also blocks presynaptic alpha-2 receptors, which increases norepinephrine release onto cardiac beta-1 receptors, on top of the baroreflex.

Which heart conditions contraindicate phentolamine?

Myocardial infarction (current or past), angina, coronary insufficiency or other coronary artery disease.

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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 - PHENTOLAMINE MESYLATE injection, powder, for solution (DailyMed (U.S. National Library of Medicine), 2024); 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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Representative model, for educational purposes only.