Type 1 antihistamines (H1 antihistamines) bind to histamine receptors in mast cells and endothelium. Medications in this group include diphenhydramine, cetirizine, hydroxyzine, fexofenadine, and loratadine. H1 antihistamines have been used in veterinary medicine for the prevention and treatment of hives, allergic rhinitis, allergic conjunctivitis, angioedema, and atopic dermatitis. Oral absorption of diphenhydramine in dogs is poor, with <10% systemic availability.1 In a study, there was no reduction in histamine-induced wheals in dogs at plasma levels that would be clinically helpful in humans.2
This double-blind crossover study investigated the effects of diphenhydramine and cetirizine on immediate and late-phase cutaneous reactions in 12 healthy laboratory beagles. Antihistamines were administered at previously recommended dosages for allergic dermatitis: 2.2 mg/kg and 2 mg/kg PO twice daily for diphenhydramine and cetirizine, respectively, for 6 days with a 2-week washout period. Histamine, compound 40/80 (positive control), and saline (negative control) were injected intradermally in the right thorax 10 days prior to drug administration as a baseline, then again on day 6, then 10 days after final drug administration.
Both immediate (20 minutes after testing) and late-phase (6 hours after testing) scores were recorded. No significant differences in wheal scores were identified between baseline and diphenhydramine administration after twice-daily administration. There was a significant decrease in wheal scores between baseline and cetirizine at both time points after twice-daily administration and no significant decrease during the return to baseline test 10 days after the last dose of cetirizine. This suggested that cetirizine (2 mg/kg PO every 12 hours) is more likely to prevent and treat cutaneous allergic reactions as compared with PO diphenhydramine.