In drug action, what does a partial agonist do?

A partial agonist plays a crucial role in pharmacology by binding to a receptor and inducing a response that is less than full activation, which distinguishes it from a full agonist that activates the receptor completely. When a partial agonist connects to a receptor, it activates it to a certain degree but does not elicit the maximum response possible, hence the term "partial."

This characteristic allows partial agonists to be utilized in situations where it is beneficial to achieve a moderate effect without overwhelming stimulation of the receptor. For instance, in cases where full receptor stimulation might lead to adverse effects, a partial agonist can provide therapeutic benefits while minimizing risks.

In contrast, a full agonist would bind and fully activate the receptor, leading to a maximal response, which may not always be desirable. A receptor antagonist, on the other hand, would block all activity at the receptor site without activating it, which is not the role of a partial agonist. Thus, "partially activates the receptor or enzyme" accurately captures the unique function of partial agonists in drug action.