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".
The IUPHAR has stated that 'potency' is "an imprecise term that should always be further defined",[1] for instance as ${\displaystyle {\ce {EC_{50))))$, ${\displaystyle {\ce {IC_{50))))$, ${\displaystyle {\ce {ED_{50))))$, ${\displaystyle {\ce {LD_{50))))$ and so on.