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).

Figure 4.
concept of potency
The concept of potency is important in determining drug dosage, however, high or low potency is significant only if the administration of an effective dose becomes impractical.