Medicine & USMLE


Sympathomimetics and Sympatholytics (New)
  1. Isoproterenol
  2. Dobutamine
  3. Norepinephrine
  4. Alpha-Methyldopa
  5. Midodrine
  6. Clonidine & Guanfacine
  7. Phenylephrine
  8. Epinephrine
  9. Dopamine


Epinephrine is a powerful sympathomimetic drug that stimulates the fight-or-flight response. At low doses, it works as a beta receptor agonist, while at high doses, it works as an alpha receptor agonist. This leads to physiologic changes like increased heart rate and increased cardiac output. At high doses, epinephrine can also cause increased blood pressure. Clinically, epinephrine is used to treat anaphylaxis, asthma, shock, and wide-angle glaucoma.

Key Points

  • Epinephrine
    • Drug Class
      • Sympathomimetic
        • mimics the effects of endogenous epinephrine secreted by the adrenal medulla
    • Site of Action
      • Beta agonist at low doses
        • Beta-1 and beta-2 receptors have higher affinity for epinephrine but get saturated quickly
        • Stronger effect on beta-2 than norepinephrine
      • Alpha agonist at high doses
        • At high doses, alpha-1 activity predominates
    • Physiologic Effects
      • ↑ BP (at high doses)
        • Vasoconstriction mediated by alpha-1 predominates and overwhelms beta-2 mediated vasodilation to increase blood pressure
      • ↑ HR
        • via beta-1 stimulation
      • ↑ CO
        • via beta-1 stimulation
    • Clinical Use
      • Treats Anaphylaxis
        • Via beta-2 mediated bronchodilation and alpha-1 mediated vasoconstriction
      • Treats Asthma
        • Via beta-2 mediated bronchodilation
      • Treats Wide-angle (open-angle) Glaucoma
        • Via alpha-1 mediated decrease in aqueous humor production
      • Treats shock
        • Via alpha-1 mediated vasoconstriction and beta-1 mediated increase in cardiac output
        • This can be useful in the treatment of cardiac arrest (e.g. part of ACLS protocol)