IC₅₀, the half maximal inhibitory concentration, represents the concentration of an inhibitor that is required for 50% inhibition of things like an enzyme, a cell, a cell receptor or a microorganism.

It's commonly used as a measure of drug effectiveness. Another measure of drug efficacy is EC₅₀ which represents the concentration of a compound that is required to obtain 50% of the maximum effect in vivo. (Ex. EC₁₀₀ would be the minimum concentration of a compound to obtain 100% of its effect).

According to the FDA, IC₅₀ represents the concentration of a drug that is required for 50% inhibition in vitro whereas EC₅₀ represents the plasma concentration required for obtaining 50% of a maximum effect in vivo.(http://www.fda.gov/ohrms/dockets/ac/00/slides/3621s1d/sld036.htm)

See also

• Dose-response relationship
• CC₅₀
• EC₅₀