full agonist

A full agonist is a ligand that, when it binds a receptor, can drive it to its maximum possible response — like a light switch that goes all the way to full brightness. It does everything the body's own natural activator would do at that receptor, reaching the full ceiling of effect the system can deliver.

Technically, a full agonist has high enough efficacy (or intrinsic activity near 1) that, given enough of it, the response plateaus at the system's maximum. This sits in a family of related behaviors: a partial agonist activates the receptor but cannot reach that ceiling no matter how much is given; an antagonist binds without activating; and an inverse agonist actively suppresses any baseline activity. The full agonist is the one that pushes the receptor all the way.

Whether a ligand acts as a full agonist is partly in the eye of the tissue. Because of receptor reserve and signal amplification, a molecule that behaves as a full agonist in a richly responsive tissue may look only partial in a tissue with few receptors. Clinically, full agonists deliver the largest effect but also leave the least headroom for further escalation and can carry the most on-target risk; partial agonists are sometimes chosen precisely because their built-in ceiling is safer.