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Background: The administration of histamine with iontophoresis is an alternative method to skin prick tests or intradermal injections. Skin reactions obtained with this method can be recorded with laser Doppler flowmetry (LDF) and previous studies with this method have shown histamine-induced laser Doppler changes in the wheal area. Objective: In order to compare the influence of two H₁ receptor antagonists (cetirizine 10 mg vs. ebastine 10 mg) on the skin vascular responses to histamine introduced by iontophoresis, we designed a double-blind, randomized, two-period crossover trial in which 18 volunteers were randomized. Methods: Before and 2, 5 and 7 h after drug administration, iontophoresis (30 s, 1.4 mA/cm²) of histamine 10% was performed and followed by (1) monitoring of skin vascular responses with LDF at the administration site and at 1 cm from it, and (2) wheal and flare area measurements. Results: 2, 5 and 7 h after intake of the antihistaminic drug, there were significant differences between both drugs. Concerning LDF recordings, we noted at the histamine administration site an increase in perfusion unit (PU) values which is an effect known to be in proportion to the degree of inhibition of wheal reaction, and at 1 cm distal to the histamine administration site, there was a decrease in PU values. These changes were more marked under cetirizine. A greater suppressive effect of cetirizine on the wheal and flare reaction was consistently observed at all time points during the study, demonstrating its superior efficacy. Conclusion: We conclude that (1) cetirizine demonstrated a stronger antihistaminic effect compared to ebastine at all time points; (2) iontophoresis appears to be an appropriate method to study specific microvascular changes at the delivery site of histamine and hence to detect the earliest changes occurring at the site of agonist-antagonist competition in the skin.

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