Potency is the dose needed, efficacy is the maximum effect achievable
Expansion
- Potency is the dose required for a given effect, measured by the EC50. A more potent drug sits further left on the dose response curve
- Efficacy is the maximum achievable effect, the Emax. A more efficacious drug reaches a higher plateau
“Potency moves the curve left; efficacy raises the ceiling.”
Efficacy matters clinically and potency usually does not: furosemide is less potent than bumetanide but equally efficacious, needing a larger milligram dose. Comparing drugs by milligrams alone is therefore meaningless.
Partial agonists have lower efficacy whatever the dose, because they cannot produce a maximal response even at full receptor occupancy. Buprenorphine is the example, which is why it precipitates withdrawal in someone dependent on a full agonist: it displaces the full agonist while producing less effect.
Antagonists
- Competitive: shifts the curve right, reducing apparent potency, Emax unchanged, and surmountable with more agonist
- Non-competitive or irreversible: reduces Emax, and cannot be overcome
Therapeutic index, the ratio of toxic to effective dose, is what determines the need for monitoring, not potency.