Partial agonist

Think of a light switch: a normal messenger from the body turns the light fully on. A partial agonist works in a similar way, but it's more like a dimmer. It binds to a docking site on the cell surface (a so-called receptor) and activates it, but only to a degree. Even when every docking site is occupied, it only lets the light come on partway, no matter how far you turn it. So it never reaches the maximum effect that a substance which fully activates the receptor (a full agonist) can achieve.

The twist

The twist: when a lot of the body's own messenger is already present, a partial agonist can even act dampening, like a blocker called an antagonist. This happens because it displaces these stronger molecules from the docking site without itself triggering an equally strong signal. So it sits "in between".

In everyday life this principle shows up in, for example, certain pain or addiction medications, which act on the same receptor as the body's own messengers, but more gently.