A histamine agonist is a drug which causes increased activity at one or more of the four histamine receptor subtypes.
H2: Betazole and Impromidine are examples of agonists used in diagnostics to increase histamine.
H3: Betahistine is a weak Histamine1 agonist and a very strong antagonist of the Histamine3 autoreceptor. Antagonizing H3 increases histaminergic tone.