EC50

The term half maximal effective concentration (EC₅₀) refers to the concentration of a drug, antibody or toxicant which induces a response halfway between the baseline and maximum after a specified exposure time. It is commonly used as a measure of drug's potency.

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.

It is also related to IC₅₀ which is a measure of a compound's inhibition (50% inhibition). 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. Sometimes it is also expressed as pEC₅₀ = - LOG(EC₅₀) (with EC₅₀ in M/l).

A small change in ligand concentration typically result in rapid changes in response in the biological system, following a sigmoidal function.The inflection point at which the increase in response with increasing ligand concentration begins to slow is the EC₅₀. Which can be mathematically determined by derivation of the best-fit line. While relying on a graph for estimation is more convenient, this typical method yields less accurate results and less precise.