In a double-blind study performed in 81 healthy volunteers, 10 mg cetirizine and 60 mg terfenadine given orally in a single administration significantly inhibited skin reactivity to histamine. Astemizole (10 mg) was completely ineffective. The inhibitory effect of cetirizine was potent and regular whereas 6/28 (21%) volunteers did not respond to terfenadine. The difference observed beween cetirizine and terfenadine might be due to differences in the metabolism of the two drugs after administration: terfenadine is rapidly and extensively metabolized whereas cetirizine is directly active without the need for biotransformation and, indeed is poorly metabolized.
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