Potency and Efficacy in Dose-Response

Potency and efficacy are the two principal parameters that summarise a graded dose-response curve, and they describe different things. Potency refers to the amount of drug needed to produce a given effect - reflected in how far left or right the curve sits on the dose axis - while efficacy refers to the maximal effect the drug can produce, reflected in the height of the curve's plateau.

Definition

Potency is the dose or concentration of a drug required to produce a specified effect, conventionally indexed by the half-maximal value (EC50 or ED50); efficacy is the maximal effect (Emax) a drug can elicit in a given system, and intrinsic efficacy is the per-receptor capacity of the agonist to generate response.

Scope

This topic distinguishes potency from efficacy, relates each to a feature of the dose-response curve, separates clinical (observed) efficacy from intrinsic efficacy at the receptor, and explains why a more potent drug is not necessarily more effective. It is reference-educational and provides no dosing guidance.

Core questions

• What is the difference between potency and efficacy?
• Which feature of a dose-response curve reflects potency, and which reflects efficacy?
• Why is a more potent drug not necessarily more efficacious?
• How does intrinsic efficacy at the receptor relate to the observed maximal effect?

Key concepts

• Potency (EC50 / ED50)
• Efficacy (Emax)
• Intrinsic efficacy
• Full versus partial agonist
• Curve position versus curve height
• Relative potency between drugs
• Spare receptors

Key theories

Operational model of agonism

Black and Leff modelled agonist response without separately assuming a linear occupancy-effect relationship, introducing a transducer function whose parameters capture the system's capacity to convert receptor occupancy into response and thereby distinguish an agonist's affinity from its efficacy.

Mechanisms

Potency depends on how readily a drug occupies and activates its target and on factors that govern the concentration reaching the target; it is read from the horizontal position of the dose-response curve, with a lower EC50 indicating higher potency. Efficacy depends on the drug's ability, once bound, to produce a response and on the responsiveness of the system; it is read from the height of the plateau (Emax). A full agonist can drive the system to its maximal response, whereas a partial agonist has lower efficacy and a lower ceiling even at full occupancy. Because potency and efficacy are independent, two drugs can differ in one without the other: a highly potent drug acting at low concentrations may have modest maximal effect, while a less potent drug may reach a higher maximum. The operational model of Black and Leff formalises how receptor occupancy is transduced into response, separating affinity from efficacy, and IUPHAR terminology fixes the definitions of these quantities.

Clinical relevance

Distinguishing potency from efficacy clarifies how drugs are compared: differences in potency mainly affect the concentration scale, whereas differences in efficacy affect the achievable maximal response. This entry presents the concepts for educational reference and is not a basis for comparing or selecting therapies for individual patients.

History

The separation of affinity from efficacy was a central problem in twentieth-century receptor theory: occupancy alone could not explain why some agonists reached a smaller maximum than others. Furchgott's notion of intrinsic efficacy and Black and Leff's 1983 operational model gave the field quantitative ways to express efficacy independently of potency, a distinction later codified in IUPHAR terminology.

Key figures

• James Whyte Black
• Paul Leff
• Terry Kenakin
• Robert Furchgott

Seminal works

• black-leff-1983
• neubig-2003

Frequently asked questions

Does a more potent drug work better than a less potent one?

Not necessarily. Potency only describes the concentration needed to produce an effect; a highly potent drug acts at low concentrations but may have a lower maximal effect (efficacy) than a less potent drug that can drive the system to a higher ceiling.

How are potency and efficacy seen on a dose-response curve?

Potency is the horizontal position of the curve - a curve further to the left (lower EC50) is more potent - while efficacy is the height of the plateau, with a taller maximum indicating greater efficacy.