The ^-receptor antagonists for the most part are substituted ethylamine compounds. In comparison with his-tamine, the ^-antagonists contain no imidazole ring and have substituents on the side chain amino group.
The ^-antagonists are classified as either first- or second-generation compounds. Second-generation anti-histamines have lipophilicity and ionization profiles that make them less able to cross the blood-brain barrier; thus they produce dramatically less sedation than do the first-generation drugs.
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