Cetirizine and loratadine: a comparison using the ED50 in skin reactions

To quantify objectively the comparative potencies of the antihistamines, loratadine and cetirizine, we determined the dose that inhibits histamine-induced skin reactions by 50% of the maximum response (ED50) for each drug. Cetirizine at 2.5, 5 or 10 mg, loratadine at 10, 20 or 40 mg or placebo were given to 14 healthy female subjects in a randomized double-blind crossover design. Inhibition of the wheal and flare response to the histamine prick test (10, 100 and 500 mg/ml) was evaluated. Depending on the histamine concentrations, the ED50s for wheals were in the ranges 4.3 - 4.7, 2.1 - 2.2 and 1.7 - 1.9 mg cetirizine, 2, 4 and 6 h after dosing, respectively. For loratadine, the ED50 for wheals were in the ranges 35.6 - > 40, 9.1 - 24.1 and 9.1 - 13.9 mg, 2, 4 and 6 h after dosing, respectively. Calculation of the ED50 demonstrated that, on average, cetirizine is seven to nine times more potent than loratadine at inhibiting wheal and flare reactions.