Duration of inhibition of immediate skin test reactivity by hydroxyzine hydrochloride in dogs

The duration of hydroxyzine-mediated suppression of the immediate hypersensitivity reaction in the skin of dogs was assessed by intradermal administration of various dilutions of histamine phosphate and of aqueous flea antigen in 18 dogs known to be allergic to fleas. Wheal diameters and scores were used to evaluate the strength of the resulting reactions. In most dogs, significant (P less than 0.05) inhibition lasted from 3 to 5 days after withdrawal from treatment. Some dogs took up to 9 days to equal or exceed their pretreatment wheal diameters and scores.