Potency (pharmacology)

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Semi-log plots of two agonists with different Kd.

In the field of pharmacology, potency is a measure of drug activity expressed in terms of the amount required to produce an effect of given intensity. A highly potent drug (e.g., morphine, alprazolam, chlorpromazine) evokes a larger response at low concentrations, while a drug of lower potency (ibuprofen, acetylsalicylic acid) evokes a small response at low concentrations. It is proportional to affinity and efficacy.

Affinity is the ability of the drug to bind to a receptor. Efficacy is the relationship between receptor occupancy and the ability to initiate a response at the molecular, cellular, tissue or system level. The response is equal to the effect, or (E), and depends on both the drug binding and the drug-bound receptor then producing a response; thus, potency depends on both affinity and efficacy. The agonist, the ligand, drug or hormone that binds to the receptor and initiates the response is usually abbreviated A or D. Below a certain concentration of agonist ([A]), E is too low to measure but at higher concentrations it becomes appreciable and rises with increasing agonist concentration [A] until at sufficiently high concentrations it can no longer be increased by raising [A] and asymptotes to a maximum Emax.

The Emax is the maximum possible effect for the agonist. The concentration of A at which E is 50% of Emax is termed the half maximal effective concentration and is abbreviated [A]50, or more commonly EC50. The term "potency" refers to the [A]50 value. The lower the [A]50, the less the concentration of a drug is required to produce 50% of maximum effect and the higher the potency.

Higher potency does not necessarily mean more side effects.


  • Walker MG, Page CP, Hoffman BF, Curtis M (2006). Integrated Pharmacology (3rd ed.). St. Louis: Mosby. ISBN 0-323-04080-2.