a substance that binds to a receptor but fails to produce the same degree of response as a full agonist at the same receptor site or exerts only part of the action exerted by the endogenous neurotransmitter that it mimics. Partial agonists may exhibit the same affinity for the receptor site as do full agonists and may act as competitive inhibitors of full agonists. Minor variations in the chemical structure of either the receptor or the binding substance may dictate whether the substance acts as a full or partial agonist at any particular receptor site.