The term EC₅₀ represents the concentration of a compound where 50% of its effect is observed.

It is commonly used as a measure of drug potency.

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

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)

Concentration measures typically follow an S-shaped curve, increasing rapidly over a relatively small change in concentration. The point at which the effectiveness slows with increasing concentration is the IC₅₀. This can be determined mathematically by derivation of the best-fit line. However, it is more easily observed from a graph and estimated rather than through complex calculus equations.

See also

• Dose-response relationship
• IC50

External links

• S-shaped curves
  
• Determination of IC₅₀ values

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