Pharmacology glossary
Potency vs efficacy: position and maximum on a concentration-response curve
Potency describes how much concentration is associated with a defined response in an assay; efficacy describes the maximal response a ligand can produce in that system. They are different properties and neither alone establishes clinical value.
Educational content only. Not medical advice.
Potency describes curve position
A more potent ligand reaches a defined response at a lower tested concentration in the same system. EC50 is often used as an operational potency measure for agonists, but its value depends on curve fitting, system amplification, receptor density, timing, and the compound's own maximum. Potency comparisons require the same assay and response definition.
Efficacy describes the achievable system response
Efficacy concerns the capacity to produce a response, commonly summarized by the observed maximum relative to a reference. A partial agonist can be highly potent yet have lower maximal efficacy. A full agonist can be less potent but reach the system maximum when concentration increases within the tested range.
Biological systems reshape both measures
Receptor reserve and signal amplification can make low occupancy sufficient for maximal response, shifting apparent potency and hiding intrinsic efficacy differences. Desensitization, biased signaling, cell background, and measurement window can further alter curves. A single assay value is an operational result, not a context-free molecular constant.
Neither measure ranks overall benefit
Higher potency does not imply greater effectiveness, safety, duration, or therapeutic value. Clinical outcomes depend on exposure, target engagement, selectivity, tissue distribution, downstream effects, adverse events, and the condition studied. Evidence summaries should avoid using potent as a synonym for powerful, superior, or proven.
Evidence limits
- Potency and efficacy estimates depend on assay conditions and curve quality.
- EC50 can shift without a change in binding affinity because signaling systems amplify responses.
- In vitro potency does not rank clinical benefit or safety.
Sources and further reading
These sources ground the definitions and evidence boundaries on this page. A citation is a route for verification, not an endorsement of a product or personal use.
StatPearls, NCBI Bookshelf
Pharmacodynamics
Authoritative overview of receptors, agonism, antagonism, affinity, efficacy, potency, and response.
Open sourceAssay Guidance Manual, NCBI Bookshelf
Assay Operations for SAR Support
Authoritative guidance on concentration-response curves, curve fitting, variability, EC50, and IC50.
Open sourceAssay Guidance Manual, NCBI Bookshelf
Glossary of Quantitative Biology Terms
Institutional definitions for EC50, IC50, assay selectivity, accuracy, controls, and quantitative biology terms.
Open sourceCommon questions
Is lower EC50 always better?
No. It indicates greater operational potency in that assay, not better outcomes or safety.
Can a partial agonist be more potent than a full agonist?
Yes. It may act at lower concentration while still producing a lower maximal response.
Is efficacy the same as effectiveness in a clinical trial?
Not here. Receptor efficacy is a pharmacologic property; clinical efficacy concerns outcomes in a defined trial.
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