Affinity, efficacy and potency

In the laboratory, affinity can be measured as the concentration of a drug that occupies 50% of the available receptors, as suggested by the definition 

A measure of the magnitude of the effect once the drug is bound. 

A measure of the quantity of the drug needed to produce maximal effect. 

Potency is compared using the median effective concentration (EC50) or median effective dose (ED50), the meanings of which are subtly different. 

The concentration of a drug that induces a specified response exactly halfway between baseline and maximum. 

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