Is Antihistamines (Benadryl, hydroxyzine) toxic to dogs or cats?

Plain diphenhydramine (Benadryl) and hydroxyzine are sometimes prescribed for dogs by vets, with a therapeutic dose of roughly 2-4 mg/kg. The real dangers are overdose and combination products: too much causes the opposite of sedation (agitation, racing heart, tremors, seizures), and many cold and allergy tablets also contain a decongestant (pseudoephedrine) or acetaminophen, which are separately toxic. Never give a combination product.

Toxicity is dose-dependent. Your dog's weight and the quantity consumed are both important - include these when you call poison control.

Antihistamines including hydroxyzine are used in cats under veterinary direction - hydroxyzine in particular is prescribed for feline allergic skin disease. But cats are sensitive to overdose and to the decongestants and pain relievers in combination cold/allergy products. These are not safe to give from the human medicine cabinet; only at a vet-determined dose.