Potency

Potency is the degree to which a substance—such as a drug, toxin, or biological agent—can achieve a given effect at a certain dose. In pharmacology, potency describes the amount of drug required to produce a defined response; a more potent compound achieves the same effect at a lower dose than a less potent one. Potency is distinct from efficacy, which is the maximal effect a compound can produce regardless of dose. A drug with high potency may produce a large effect at low concentrations but not necessarily have a larger maximum effect than a less potent drug.

Potency is commonly quantified using dose–response curves. In vitro, the EC50 is the concentration that produces 50% of the maximal response; in vivo, the ED50 is the dose that achieves 50%. Lower EC50 or ED50 indicates higher potency. The shape and position of the curve depend on factors such as receptor affinity, intrinsic activity, and pharmacokinetics.

Several scenarios affect potency. Competitive antagonists reduce apparent potency by shifting the dose–response curve to higher doses without changing the maximum effect. Noncompetitive antagonists or irreversible inhibition can reduce efficacy as well as potency. Pharmacokinetic factors—absorption, distribution, metabolism, and elimination—also influence the dose needed to reach target concentrations.

Potency is context-dependent and relative, useful for comparing similar agents under standardized conditions. It should not be assumed to reflect safety or therapeutic value; a highly potent drug may have a narrow therapeutic window or more adverse effects, and higher potency is not inherently better.

Beyond pharmacology, potency can describe the strength of a chemical’s biological effect or an extract’s active content, though the precise meaning varies by field.