Background and objective : Non-sedating antihistamines (loratadine, fexofenadine, and cetirizine) have been widely used in Thailand. This study examined the time-of-onset and compared the 95% inhibitory effect of these agents on histamine-induced cutaneous reaction so as to understand the diversity of their efficacy.
Patients and Method : Thirty-one atopic patients were randomized into 4 treatment groups: placebo (n = 7), loratadine (n = 8), fexofenadine (n = 8), and cetirizine (n = 8). They were pricked with histamine every 30 min for 4 hrs. The percentage change of the wheal/flare area was calculated.
Results : All active treatments showed wheal suppression superior to placebo after 210 min for loratadine (P = 0,04); 90 min for fexofenadine (P = 0,009); and 60 min for cetirizine (P = 0,02), while flare suppression was significantly marked after 150 min (P = 0,0008) for loratadine; 90 min for fexofenadine (P = 0,0001); nd 60 min for cetirizine (P = 0,006). All drugs except loratadine demonstrated a 95% suppression of wheal uperior to the placebo (P = 0,001 for fexofenadine; P = 0,0001 for cetirizine). Only fexofenadine exhibited a 95% suppression of flare statistically superior to placebo (P = 0,02). Discrepancies among the effects of these 3 antihistamines were also detected.
Discussion and Conclusion : All antihistamines tested repressed the wheal-and-flare area superbly over the placebo within 4 hours. Cetirizine exerted the fastest onset, and it also appeared to be the most efficacious inhibitor. The heterogeneity of their efficacy probably stems from their diverse physicochemical properties, which have also been discussed.

Keywords : Wheal-and-Flare Suppression, Non-Sedating Antihistamines