Pharmacologic modulation by cetirizine 2 HCl and loratadine of the histamine‐induced skin reaction in mice and in humans

In mice, the histamine‐induced cutaneous reaction is inhibited by cetirizine with an approached ED 50 of 0.04 mg/kg and by loratadine with an approached ED 50 of 0.38 mg/kg, both drugsadministered intraperitoneally. At these doses, cetirizine' s onset of activity seems to be more rapid, although both compounds show the same duration for at least 7 hr. Nine healthy volunteers received orally either cetirizine or loratadine (10 mg single dose) or placebo in a double‐blind randomized and cross‐over study design. Both drugswere significantly more effective than placebo (except loratadine at the 2nd and 24th hr for the wheal with 1 and 10 μg histamine, and at the 2nd hr for the flare with 1 μg histamine). At all times and withboth agonists' concentrations, cetirizine was significantly more potent than loratadine (except at the 24th hr for the wheal with 1 μghistamine). Neither numerous nor marked side effects were found.