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One of unwanted side effects of older antihistamines is their sedative action. Some groups of antihistamines have anticholingergic activity and antiserotonin activity in addition to sedation. Synthesis of newer antihistamines have been undertaken in order to diminish these unwanted side effects. Terfenadine and Astemizole were both synthesized in order to have a greater specific activity on peripheral H1-histaminergic receptors than central H1-histaminergic receptors. Therefore, Terfenadine and Astemizole do not cause as much sedation and other CNS side effects as the older antihistamines. However, Mequitazine, one of the new antihistamines can cause CNS side effects at a dose of twice the therapeutic dose. For this reason it is no more popular than the older antihistamines.
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