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. A highly potent drug (e.g., fentanyl, alprazolam, risperidone) evokes a given response at low concentrations, while a drug of lower potency (meperidine, diazepam, ziprasidone) evokes the same response only at higher concentrations. Higher potency does not necessarily mean more side effects.
The IUPHAR has stated that 'potency' is "an imprecise term that should always be further defined", for instance as , , ED50, LD50 and so on.