What is the difference between EC50 and IC50?

The concepts of EC₅₀ and IC₅₀, albeit similar, are not quite identical. While EC₅₀ measures the concentration of a drug inducing its half-maximal effective response, IC₅₀ represents the concentration of an inhibitor at which 50% of inhibition in its activity is achieved. A good way to remember the difference is using the acronym ‘I’ in IC₅₀, which stands for inhibition, unlike ‘E’ in EC₅₀, which refers to effective.