What is the difference between potency and efficacy?

Potency is how much drug is needed to produce a given effect (the position of the dose-response curve), whereas efficacy is the maximal effect the drug can produce (the height of the curve). A drug can be very potent but have low efficacy, or vice versa.

Does higher potency mean a better drug?

No. Potency only describes the concentration needed for an effect; it says nothing about the maximal effect achievable, selectivity, or safety, all of which matter when characterising a drug's overall profile.

Efficacy and Potency

Efficacy and potency are two distinct but often-confused properties of a drug. Efficacy is the maximal effect a drug can produce, no matter how high the dose; potency is the amount of drug needed to produce a given level of effect. Two drugs can be equally efficacious yet differ greatly in potency, and the more potent drug is not necessarily the more useful one.

Definition

Potency is the concentration or dose of a drug required to produce a specified effect (often half of its maximal effect), while efficacy is the maximal effect that the drug is capable of producing once bound to its target.

Scope

This topic distinguishes efficacy (including intrinsic efficacy) from potency, relates them to the dose-response curve, and explains why each is governed by different molecular factors. It is a reference and educational entry and offers no comparative claims about specific products or prescribing guidance.

Core questions

How do efficacy and potency differ conceptually and on a dose-response curve?
Which molecular factors determine potency, and which determine efficacy?
Why is a more potent drug not necessarily more effective or preferable?
What is intrinsic efficacy and how does it relate to agonist behaviour?

Key concepts

Potency (EC50 / ED50)
Efficacy and maximal effect (Emax)
Intrinsic efficacy
Affinity versus efficacy
Receptor reserve (spare receptors)
Full versus partial agonism
Position versus height of the dose-response curve

Key theories

Efficacy as a separate parameter from affinity: Stephenson extended occupancy theory by introducing efficacy as a property distinct from affinity, proposing that a maximal response can be reached without full receptor occupancy and that agonists differ in the stimulus they generate per receptor occupied; this separated potency (driven largely by affinity and coupling) from efficacy (the capacity to produce response).
Operational quantification of efficacy: Black and Leff captured efficacy operationally through a transducer ratio that expresses how effectively agonist-receptor occupancy is converted into response in a given tissue, allowing efficacy and affinity to be estimated separately from concentration-response data.

Mechanisms

On a graded dose-response curve, potency corresponds to the horizontal position of the curve (its half-maximal effective concentration), while efficacy corresponds to the height of its plateau (the maximal effect). Potency depends on the drug's affinity for its target and on how efficiently target activation is coupled to response, including any receptor reserve. Efficacy depends on intrinsic efficacy - the strength of the stimulus generated per occupied receptor - so a full agonist has high efficacy and a partial agonist lower efficacy regardless of how many receptors it occupies. Because potency and efficacy are governed by different factors, they vary independently across drugs acting at the same target. International nomenclature provides standardised symbols (such as EC50 and Emax) for reporting them.

Clinical relevance

Distinguishing potency from efficacy explains why a drug that acts at a very low concentration is not automatically superior, and why the achievable maximal effect can matter more than the dose required to reach it. This entry is educational and conceptual; it makes no recommendations about choosing or dosing specific medicines.

Evidence & guidelines

The IUPHAR Committee on Receptor Nomenclature and Drug Classification defines and standardises the terms efficacy, intrinsic efficacy, potency, EC50, and Emax, providing the agreed vocabulary used to report these parameters in pharmacology.

History

Early occupancy theory equated effect with receptor occupancy, which could not explain why some agonists reached a maximal response without full occupancy. Ariens introduced intrinsic activity and Stephenson introduced efficacy in the 1950s to resolve this, separating a drug's ability to produce a response from its affinity for the receptor. Black and Leff's 1983 operational model then provided a way to estimate efficacy and affinity independently, consolidating potency and efficacy as separate, measurable parameters.

Key figures

Robert Stephenson
Everardus Ariens
James Black
Terry Kenakin

Seminal works

stephenson-1956
black-leff-1983
neubig-2003

Frequently asked questions

What is the difference between potency and efficacy?: Potency is how much drug is needed to produce a given effect (the position of the dose-response curve), whereas efficacy is the maximal effect the drug can produce (the height of the curve). A drug can be very potent but have low efficacy, or vice versa.
Does higher potency mean a better drug?: No. Potency only describes the concentration needed for an effect; it says nothing about the maximal effect achievable, selectivity, or safety, all of which matter when characterising a drug's overall profile.