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In 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 evokes a larger 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 [A]50, or 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.
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