<P> Loratadine is a tricyclic antihistamine, which acts as a selective inverse agonist of peripheral histamine H - receptors . The potency of second generation histamine antagonists is (from strongest to weakest) desloratadine (K 0.4 nM)> levoceterizine (K 3 nM)> cetirizine (K 6 nM)> fexofenadine (K 10 nM)> terfenadine> loratadine . However, the onset of action varies significantly and clinical efficacy is not always directly related to only the H receptor potency . </P> <P> Loratadine is given orally, is well absorbed from the gastrointestinal tract, and has rapid first - pass hepatic metabolism; it is metabolized by isoenzymes of the cytochrome P450 system, including CYP3A4, CYP2D6, and, to a lesser extent, several others . Loratadine is almost totally (97--99%) bound to plasma proteins . Its metabolite desloratadine, which is largely responsible for the antihistaminergic effects, binds to plasma proteins by 73--76% . </P> <P> Loratadine's peak effect occurs after one to two hours, and its biological half - life is on average 8 hours (range 3 to 20 hours) with desloratadine's half - life being 27 hours (range 9 to 92 hours), accounting for its long - lasting effect . About 40% is excreted as conjugated metabolites into the urine, and a similar amount is excreted into the feces . Traces of unmetabolised loratadine can be found in the urine . </P> <P> In structure, it is closely related to tricyclic antidepressants, such as imipramine, and is distantly related to the atypical antipsychotic quetiapine . </P>

How long does it take for loratidine to work
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