Cetirizine. Molecular model of the second-generation antihistamine drug cetirizine (C21.H25.Cl.N2.O3). A second-generation antihistamine does not cross the blood?rain barrier and acts mainly outside the central nervous system. Antihistamine drugs counteract the effect of histamine, a body chemical that is responsible for many symptoms of an allergic response. Atoms are represented as spheres and are colour-coded: carbon (grey), hydrogen (white), nitrogen (blue), oxygen (red) and chlorine (green).