Medicine & USMLE


Alpha & Beta Blockers (New)
  1. Phentolamine


Phentolamine is a drug that works as a non-selective alpha blocker at both alpha-1 and alpha-2 receptors. The effects of phentolamine are reversible.

Clinically, the drug is used to lower blood pressure in order to treat hypertensive emergencies. For example, phentolamine can be used to treat the hypertensive emergency caused by tyramine in patients taking MAO inhibitors. Phentolamine may also be used to treat hypertensive emergencies caused by cocaine. In addition, the drug can also be used to treat norepinephrine extravasation.

Side effects of phentolamine include orthostatic hypotension and reflex tachycardia.

Key Points

  • Phentolamine
    • Mechanism
      • Blocks Alpha-1 and Alpha-2 Receptors (Nonselective)
        • Reversible
          • Compare vs. phenoxybenzamine
        • Most pharmacologic effects mediated by alpha-1 receptor blockade, which prevents vasoconstriction
    • Clinical Use
      • Treats Hypertensive Crises
        • Treats hypertensive crisis caused by tyramine in patients taking MAO inhibitors
          • Consuming foods rich in tyramine can lead to excessive catecholamine production in these patients, leading to hypertension
        • Treats cocaine-induced hypertension
        • Treats Pheochromocytomas
          • Phenoxybenzamine (irreversible) is preferred
      • Treats NE (norepinephrine) extravasation (skin necrosis)
        • Norepinephrine can cause extreme vasoconstriction, leading to skin necrosis around the IV site, which can be reversed by phentolamine
    • Side Effects
      • Causes Orthostatic Hypotension
        • Blocks Alpha 1 receptors→ Decreases venous vasoconstriction → decreases venous return against the effects of gravity from standing up
      • Causes Reflex Tachycardia
        • Causes a decrease in diastolic blood pressure and reflex tachycardia