Join Date: Apr 2009
Location: West Chester, PA
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This is from a recent post I made on the health forum:
"Efficacy of antihistamines in dogs is generally not too good and highly variable. What works in one dog may not do squat in another. Clemastine (Tavist) works for 30% and cetirizine (Zyrtec) for 18%. For reasons not understood, antihistamines in dogs chemically degrade quickly so you can give large doses. Talk to your vet or dermatologist to see how much you can safely give. Rocky's dermatologist said 1.34mg clemastine 2-3x per day was fine. According to a study in the Canadian Veterinary Journal, cetirizine protocols vary widely from 2.5-10mg/dog, q24h, up to 1.1 mg/kgBW, q12h. This particular study used 1mg/kgBW, q24h, which did make 2 of the 22 dogs mildly sick for the first day or two although it appears that the vomiting in one of those two dogs was caused by an unrelated condition. Hydroxyzine is available from your vet (Rx only). It did nothing for Rocky."
The title of the paper is "Treatment of canine atopic dermatitis with cetirizine, a second generation antihistamine: A single-blinded, placebo-controlled study" published in May 2004.
Based on this paper, I would say 1mg/kg of body weight once per day would be completely safe but I am NOT a vet!!!
I believe clemastine (Tavist) has the highest success rate - surprised your vet didn't mention this one.
PS. cetirizine hydrochloride (Zyrtec) is a metabolite of hydroxyzine.