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. Examples include histamine receptor agonists and histamine receptor antagonists (or antihistamines). Subdivisions of histamine antagonists include H<sub>1</sub> receptor antagonists, H<sub>2</sub> receptor antagonists, and H<sub>3</sub> receptor antagonists.