Understanding the difference between efficacy and potency is key in pharmacology and often tested on USMLE Step 1.

Efficacy

• Definition: The maximal effect a drug can produce (intrinsic activity).
• Graph representation: Reflected in the Y-axis (Emax).
  • ↑ Y-value = ↑ Emax = ↑ efficacy
• Key Points
  • Unrelated to potency (a drug can be very efficacious but require high doses, or have low potency but still high efficacy).
  • Partial agonists have lower efficacy than full agonists, regardless of dose.

Potency

• Definition: The amount of drug needed to achieve a given effect.
• Graph representation: Reflected in the X-axis (EC₅₀)
  • Left shift = ↓ EC₅₀ = ↑ potency (less drug needed).
  • Right shift = ↑ EC₅₀ = ↓ potency (more drug needed).
• Key Points
• Unrelated to efficacy (a potent drug can have a high or low maximal effect).
• EC = Effective Concentration.

Comparison Table

Learning Objective :

Be able to distinguish efficacy from potency on a dose-response curve:

• Efficacy = how effective the drug is at its maximum (Y-axis).
• Potency = how much drug is needed to get there (X-axis).