A histamine agonist is a drug which causes increased activity at one or more of the four histamine receptor subtypes.
H<sub>1</sub> agonists promote wakefulness.
H<sub>2</sub>: Betazole and Impromidine are examples of agonists used in diagnostics to increase histamine.
H<sub>3</sub>: Betahistine is a weak Histamine<sub>1</sub> agonist and a very strong antagonist of the Histamine<sub>3</sub> autoreceptor. Antagonizing H<sub>3</sub> increases histaminergic tone.