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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.
Histamine-1 receptors are located in the airways, blood vessels and gastrointestinal tract (stomach and esophagus).
Stimulation of these receptors can lead to conditions such as a skin rash or inflammation, a narrowing of the airways (bronchoconstriction), hay fever, or motion sickness.
They have a longer duration of action and longer plasma half-life than the first generationdrugs, which makes it possible to reduce the number of daily intakes.
They do not have, in normal dose, antimuscarinic effect, except mequitazine which has a mild antimuscarinic effect.
Antihistamines are inhibitors of histamine receptors. H1-antihistamines, inhibit competitively H1 receptors and the corresponding effects i. They do not inhibit antigen/antibodies reactions, nor histamine release, they inhibit H1 effects.
H1-antihistamines have been used for more than fifty years in treating various allergic manifestations. H1-antihistamines which penetrate into brain elicit, by inhibiting stimulant effect of histamine, drowsiness.
H1-antihistamines are used for supportive care of allergic manifestations, cutaneous (urticaria) or mucous membranes (rhinitis, hayfever, conjunctivitis). Insufficient alone to treat anaphylactic shock or edema of the larynx, they could prevent them.
Promethazine has been prescribed to pregnant women without inducing malformations.
Antihistamines are a class of agents that block histamine release from histamine-1 receptors and are mostly used to treat allergies or cold and flu symptoms, although some first-generation antihistamines may also be used for other conditions.
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They were withdrawn from the market because they could induce ventricular arrhrythmias, prolongation of QT interval which can lead to torsades de pointes.