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Concentration-response curves illustrating the concept of potency. For a response of 0.25a.u., Drug B is more potent, as it generates this response at a lower concentration. For a response of 0.75a.u., Drug A is more potent. a.u. refers to "arbitrary units".

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.^[1] A highly potent drug (e.g., fentanyl, alprazolam, risperidone) evokes a given response at low concentrations, while a drug of lower potency (codeine, diazepam, ziprasidone) evokes the same response only at higher concentrations. The potency depends on both the affinity and efficacy.

Affinity is how well a drug can bind to a receptor (Fast/strong binding = higher affinity). Efficacy is the relationship between receptor occupancy and the ability to initiate a response at the molecular, cellular, tissue or system level. In other words, efficacy refers to how well an action is took after the drug is bound to a receptor. In pharmacology, a high efficacy usually means that a drug has worked since the drug caused the receptor to metabolize a certain compound extremely well. Therefore, it makes sense that a drug's effectiveness, potency, is affected by how well the drug can bind to a receptor, affinity, and how it is able to cause a reaction in the receptor when bound, efficacy.

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 E_max.

The E_max is the maximum possible effect for the agonist. The concentration of A at which E is 50% of E_max 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.

The pharmacophore is the part of the drug molecule - the atoms and groups - that bind to the receptor; the "auxophore" are the parts of the molecule that are not directly involved in binding, but may rather interfere with binding, be essential for the arrangement of pharmacophoric elements, or may be irrelevant.^[2]:17

References

Further reading

• 
  Harris, Robert (2012-10-09). "Formulating High Potency Drugs". Contract Pharma. Retrieved 2013-11-13.
  


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