histaminergic
Histaminergic means "working on the histamine system", and histaminic means "related to histamine". A histaminergic agent (or drug) is a chemical which functions to directly modulate the histamine system in the body or brain.
histamine antagonist
drug that binds to but does not activate histamine receptors, thereby blocking the actions of histamine or histamine agonists
histamine agonist
drug that binds to and activates histamine receptors