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