Intrinsic activity or efficacy refers to the ability of a drug to induce a biological response in its molecular target when bound. Efficacy can also be defined as the magnitude of effect after binding.

High-efficacy agonists can produce their maximal response while occupying a relatively low proportion of receptors; agonists of lower efficacy cannot activate the receptors to the same degree and may not be able to produce the same maximal response even when they occupy the entire receptor population, thereby behaving as partial agonists.The term is often used to classify the activity of a drug upon binding to its receptor. Efficacy or Emax is thus the maximal response produced by a drug.

For the different kinds of ligands this means:

  • full agonist elicits maximal efficacy (ε=1   or   ε = 100%).
  • An antagonist will not evoke any response and is thus without efficacy (ε = 0).
  • partial agonist displays an intermediate efficacy between 0 and 1 (ε  between 0 and 100%).