The following pharmacology definition has been taken from the Pharmacology and Experimental Therapeutics Department Glossary at Boston University School of Medicine.


Broadly, efficacy refers to the capacity of a drug to produce an alteration in a target cell/organ after binding to its receptor. A competitive antagonist, that occupies a binding site without producing any alteration in the receptor, is considered to have an efficacy of zero.

Efficacy is generally independent of potency/affinity, and is related to the maximum effect that a particular drug is capable of producing.

As originally formulated by Stephenson (1956), binding of an agonist A to its receptor R is considered to result in a “stimulus” S=? A x P AR where ? A is the efficacy of A and PAR is the proportion of the receptors occupied. The effect of the drug on the cell or tissue is given by Effect = f (S), where f is an unspecified monotonic function that is dependent upon the nature of the receptor and its interaction with the cell or tissue. Efficacy is both agonist and tissue-dependent.

Efficacy is related to Intrinsic Activity, which was originally defined by Furchgott (1966) as e=?/R T , i.e. as the efficacy per receptor. In practice, the two terms are sometimes loosely used synonymously

The copyright of the text is hold by Trustees of Boston University. Permission has been granted for its use in this blog.

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