Description a histamine h1 antagonist used in allergic reactions, hay fever, rhinitis, urticaria, and asthma.indication for the treatment of rhinitis, urticaria, allergy, common cold, asthma and hay fever.pharmacodynamics in allergic reactions an allergen interacts with and cross-links surface ige antibodies on mast cells and basophils. once the mast cell-antibody-antigen complex is formed, a complex series of events occurs that eventually leads to cell-degranulation and the release of histamine (and other chemical mediators) from the mast cell or basophil. once released, histamine can react with local or widespread tissues through histamine receptors. histamine, acting on h1-receptors, produces pruritis, vasodilatation, hypotension, flushing, headache, tachycardia, and bronchoconstriction. histamine also increases vascular permeability and potentiates pain. chlorpheniramine, is a histamine h1 antagonist (or more correctly, an inverse histamine agonist) of the alkylamine class.