Medicine & USMLE


Alpha and Beta Blockers (Old)
  1. Phenoxybenzamine
  2. Phentolamine
  3. Alpha-1 Antagonists (Prazosin, Terazosin, Tamsulosin)
  4. Beta-1 Selective Blockers (Atenolol, Esmolol, Metoprolol)
  5. Combined Alpha-Beta Blockers (Carvedilol, Labetalol)
  6. Non-selective Beta-Blockers (Propranolol, Timolol)
  7. Nebivolol


Phentolamine is a drug that works by reversibly blocking or antagonizing alpha-1 and alpha-2 receptors. Blockade of alpha-1 receptors specifically causes reduced vasoconstriction, reducing vascular resistance and blood pressure. This mechanism can be used to treat hypertensive tyramine crisis with use of MAO inhibitors as well as pheochromocytoma. Potential side effects include orthostatic hypotension and reflex tachycardia.

Key Points

  • Phentolamine
    • Mechanism of Action
      • Reversible blockade of Alpha 1 and 2 Receptors
        • Alpha 1 Blockade → Decreased vasoconstriction → Decreased TPR → Decreased BP
        • Phentolamine is a non-specific alpha1 and alpha2 adrenergic antagonist with no beta­-blocking effects. The general effects are vasodilatory
    • Clinical use
      • Adrenergic crises in general
        • patients on MAO Inhibitors who eat tyramine-containing foods
        • Cocaine Toxicity
        • Pheochromocytomas
          • Although phenoxybenzamine (irreversible) is preferred in treatment of pheochromocytoma
    • Adverse Effects
      • Orthostatic Hypotension
        • Alpha 1 blockade → Decreased venous vasoconstriction → Decreased venous return against the effects of gravity from standing up
        • Phentolamine causes a profound fall in diastolic blood pressure and reflex tachycardia
      • Reflex Tachycardia