EC50

The tissue response (y-axis) to an agonist, in log concentration (x-axis), in the presence of different antagonist concentrations. The EC₅₀ of the agonist is represented by the x co-ordinate that corresponds with the half-maximum of the leftmost curve. This is denoted by [A]

Half maximal effective concentration (EC₅₀) is a measure of the concentration of a drug, antibody or toxicant which induces a biological response halfway between the baseline and maximum after a specified exposure time. More simply, EC₅₀ can be defined as the concentration required to obtain a 50% [...] effect and may be also written as [A]₅₀. It is commonly used as a measure of a drug's potency, although the use of EC₅₀ is preferred over that of 'potency', which has been criticised for its vagueness. EC₅₀ is a measure of concentration, expressed in molar units (M), where 1 M is equivalent to 1 mol/L.

The EC₅₀ of a graded dose response curve therefore represents the concentration of a compound where 50% of its maximal effect is observed. The EC₅₀ of a quantal dose response curve represents the concentration of a compound where 50% of the population exhibit a response, after a specified exposure duration.

For clarification, a graded dose response curve shows the graded effect of the drug (y axis) over the dose of the drug (x axis) in one or an average of subjects. A quantal dose response curve shows the percentage of subjects where a response is noted in an all-or-none manner (y axis) over the dose of the drug (x axis).

For competition binding assays and functional antagonist assays IC₅₀ is the most common summary measure of the dose-response curve. For agonist/stimulator assays the most common summary measure is the EC₅₀.

The EC₅₀ is also related to IC₅₀ which is a measure of a compound's inhibition (50% inhibition).