Potency of a drug is defined as that concentration at which the drug elicits 50 percent of its Emax, i.e., EC50. It reflects both the affinity of a drug to its receptors as well as its intrinsic activity. Note in Figure 4 that for a given dose (x1), the effect of drug B at B1 is smaller than the effect of drug A at A1. Drug A also reaches the upper plateau (Emax) of the dose-response curve at a smaller dose, i.e., drug A is more potent than drug B, yet they have the same efficacy (Figure 4).
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