Potency and efficacy

Potency and efficacy

Two concepts that at this stage should not present us with any difficulty are the `potency' and `efficacy' of a drug. The potency is a function of the amount of drug required for its specific effect to occur; it is measured simply as the inverse of the EC₅₀ for that drug. In contrast, the efficacy measures the maximum strength of the effect itself, at saturating drug concentrations. Thus, in Figure 3.10, drug Red exceeds drug Black in potency, while the opposite is true of the efficacy.