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In a double-blind, crossover, randomized clinical pharmacological study performed on 10 healthy volunteers, peripheral and central effects of 10 mg cetirizine and 10 and 40 mg loratadine were compared. Cetirizine (10 mg) significantly (P less than 0.001) inhibited 10 or 100 mg/ml histamine-induced weals 2 and 6 h after drug intake. Cetirizine was more potent than 10 mg loratadine after 2 and 6 h, and was even more potent than 40 mg loratadine after 6 h. Neither drug affected subjective evaluation of central effects and cetirizine was completely devoid of electro-encephalographic (EEG) changes, whereas 10 and 40 mg loratadine induced only slight and limited EEG changes.

The peripheral and central effects of 10 mg cetirizine 2 HCl and 60 mg terfenadine have been compared with placebo in 9 healthy male volunteers. The peripheral effect, in terms of cutaneous reactivity to 1 microgram histamine i.d., was measured by planimetry of the wheal and erythemas. Central effects were assessed with a self-evaluation visual scale and from the results of electroencephalographic spectrum analysis. Peripheral inhibition of histamine reactivity was more intense and quicker for cetirizine than for terfenadine. On the self-evaluation scale, no significant difference between terfenadine, cetirizine and placebo was noted. The quantified EEG did not show any variation in spectral parameters at any time after cetirizine. By contrast, at 6 h terfenadine had increased slow waves and had inhibited the alpha band. Thus, 10 mg cetirizine 2 HCl had less effect on the central nervous system than terfenadine 60 mg, whilst its peripheral action appeared more quickly and was more intense.

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