Agonist

An agonist is a molecule that binds to a receptor and switches it on, producing the same kind of cellular response as the body's own signaling molecule.

Receptors are proteins that sit in or on cells waiting for a chemical signal. An agonist fits the receptor's binding site and changes its shape so that the cell responds, whether by opening an ion channel, activating an enzyme or altering gene expression. A full agonist produces the largest response the receptor can give; a partial agonist produces less even when every receptor is occupied. The body's own messengers, such as anandamide at cannabinoid receptors, are agonists by definition.

The term is useful in cannabis discussions because THC is a partial agonist at the CB1 and CB2 receptors, which accounts for its intoxicating effects. CBD, by contrast, has very low affinity for those sites and is generally described as acting indirectly or at other targets, so calling it a cannabinoid receptor agonist is inaccurate. How these differences translate into effects in people is still being studied.