Inverse Agonist

In the field of pharmacology, an inverse agonist is an agent that binds to the same receptor as an agonist but induces a pharmacological response opposite to that agonist.

A prerequisite for an inverse agonist response is that the receptor must have a constitutive (also known as intrinsic or basal) level activity in the absence of any ligand. An agonist increases the activity of a receptor above its basal level while an inverse agonist decreases the activity below the basal level. A neutral antagonist has no activity in the absence of an agonist or inverse agonist but can block the activity of either.

The efficacy of a full agonist is by definition 100%, a neutral antagonist has 0%, while an inverse agonist has < 0% (i.e., negative) efficacy.