CLASSIFICATION OF H1 ANTIHISTAMINICS- FIRST GENERATION ANTIHISTAMINICS- 1)HIGHLY SEDATIVE-DIPHENHYDRAMINE,DIMENHYDRINATE,PROMETHAZINE,HYDROXYZINE 2)MODERATELY SEDATIVE- PHENARIMINE,CYPROHEPTADINE, MECLIZINE,CINNARIZINE 3)MILD SEDATIVE-CHLORPHENIRAMINE,DEXCHLORPHENIRAMINE TRIPROLIDINE,CLEMASTINE SECOND GENERATION ANTIHISTAMINICS-FEXOFENADINE, LORATADINE,DESLORATADINE,CETIRIZINE,LEVOCETIRIZINE, AZELASTINE,MIZOLASTINE,EBASTINE,RUPATADINE. Mechanism of action of 2nd generation antihistaminics- These drugs competitively antagonize actions of histamine at the H1 receptors. Pharmacological actions- Antagonism of histamine-The H1 antagonists effectively block histamine induced bronchoconstriction, contraction of intestinal and other smooth muscle and triple response especially wheal, flare and itch. Constriction of larger blood vessel by histamine is also antagonized. 2) Antiallergic actions-Many manifestations of immediate hypersensitivity (type I reactions)are suppressed. Urticaria, itching and angioedema are well controlled.3) CNS action-The older antihistamines produce variable degree of CNS depression.But in case of 2nd gen antihistaminics there is less CNS depressant property as these cross BBB to significantly lesser extent. 4) Anticholinergic action- many H1 blockers in addition antagonize muscarinic actions of ACh. BUT IN 2ND gen histaminics there is Higher H1 selectivitiy : no anticholinergic side effects