1, the mechanism of action classification antihistamines are divided into two categories: H1 receptor antagonists and H2 receptor antagonists, the former is mainly used for anti-allergic, the latter is mainly used for anti-ulcer. H1 receptor antagonists H1 receptor antagonists (H1 receptor antagonists, H1RAS) are traditionally divided into different pharmacological effects: the first generation of H1RAS, such as chlorpheniramine, cycloheximide, hydroxyzine, etc.; the second generation of H1RAS, such as cetirizine, loratadine, imipramine, astemizole, etc.; the third generation of H1RAS, such as fexofenadine, desmethyl third-generation H1RAS, such as fexofenadine, desmethylamizole, decarboxylated cloratadine, etc. Mainly used for anti-allergy. Commonly used H1 receptor antagonists 2, structural classification: antihistamines according to the chemical structure, can be divided into: 1, monoethanolamine class: such as Benadryl, Doxylamine, Stastin 2, alkylamines: such as chlorpheniramine maleate (paracetamol), Triprolidine (Kemin) 3, ethylenediamine class: such as bendipyridamole, Methapyrine 4, phenothiazine class: such as promethazine, methaqualazine 5, piperazine class: such as hydroxyzine, dechlorinated Hydroxyzine, cetirizine, levocetirizine 6, piperidine: such as cycloheximide, loratadine, ibastine, imipramine, fexofenadine