as the affinity. Efficacy, on the other hand, is a measure of the action of a drug once binding has occurred. The maximum response, , will be reduced if efficacy is sufficiently low.

The definition of efficacy has been object for discussion. The only way in which absolute measures of efficacy have been obtained is by single ion channel analysis of ligand gated ion channels. It is still not possible to do this for G protein-linked receptors.

In the case of the glycine receptor

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