Background: Concomitant administration of an H1-receptor antagonist with an H2-receptor antagonist may enhance the wheal and flare suppression produced by the H1-antagonist. This synergism may be due, at least in part, to a pharmacokinetic effect.
Methods: In a randomized, double-blind, parallel-group study in 16 patients with chronic urticaria, we investigated the pharmacokinetics and suppressive effect on the histamine-induced wheal and flare of a single dose of hydroxyzine 25 mg or cetirizine 10 mg, given before and after treatment with cimetidine 600 mg every 12 hours for 10 days.
Results: When hydroxyzine was administered with cimetidine, the partial hydroxyzine area under the curve increased significantly (p < 0.05) to 303 +/- 92 ng/ml/hr from 227 +/- 77 ng/ml/hr after administration of hydroxyzine alone, and the concentration of cetirizine arising from hydroxyzine was lower. When hydroxyzine was given with cimetidine, wheal and flare suppression increased compared with when hydrozyzine was given alone, but the differences were not statistically significant (p > 0.05). When cetirizine was administered with cimetidine, the pharmacokinetics of cetirizine did not change significantly, and no enhancement of wheal and flare suppression was observed.
Conclusions: In this study co-administration of hydroxyzine with cimetidine resulted in significantly increased serum hydroxyzine concentrations and increased wheal and flare suppression, thus confirming the rationale for a trial of concomitant administration of these medications in some patients with chronic urticaria unresponsive to treatment with an H1-antagonist alone. We found no therapeutic rationale for co-administration of cetirizine with cimetidine in urticaria treatment. These medications may be co-administered safely without fear of medication interaction.