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The drugs of the first antihistamine generation are: promethazine, alimemazine, dexchlorpheniramine, brompheniramine, buclizine, carbinoxamine and doxylamine, are sedating and elicit a drowsiness which can be awkward.The first-genaration antihistamines have alpha adrenolytic activity which can decrease the vasoconstrictive effect of adrenaline and noradrenaline and an antimuscarinic effect with the corresponding adverse effects.The two first H1-antihistamines without sedating effect to be marketed were terfenadine and astemizole.They were withdrawn from the market because they could induce ventricular arrhrythmias, prolongation of QT interval which can lead to torsades de pointes.In case of overdose, loratadine can cause sedation and antimuscarinic effects.
All H1-antihistamines, including those which are intended for treatment of motion sickness, are disadvised during the first three months of pregnancy, more for reasons of principle than for observations of malformations.
Mequitazine has an antimuscarinic effect parallel to its H1-antihistamine activity, which can result for example in dryness of mouth or accommodation disturbances.
Loratadine acts mainly via one of its metabolites, decarboethoxyloratadine, also called desloratadine, which is also marketed.
Ceterizine is a racemic; one of its isomers called levocetirizine is also released on the market.
Mizolastine does not induce, in usual dosage, prolongation of QT interval; however its combination with macrolides and imidazol antifungal agents is disadvised because of a possible inhibition of its catabolism.