Antihistamines such as hydroxyzine (Atarax®), diphenhydramine (Benadryl®), and chlorpheniramine (Chlor-Trimeton®) are used in veterinary medicine primarily to treat allergies that cause skin problems. Less often, they are prescribed to prevent carsickness, to treat severe allergic reactions to insect bites or stings, or as sedatives.

Antihistamines are used very commonly in humans to treat allergies. The medicines tend to be less effective in dogs and cats.   However, they produce a significant benefit in many pets and have a good safety profile.  Because of this, they are commonly prescribed.

Antihistamines are sometimes prescribed to treat feline asthma.  Most experts believe that this is not an appropriate use for antihistamines.

Loratadine (Claritin®) and fexofenadine (Allegra®) are not frequently used in veterinary medicine.


Antihistamines are readily available as pills and liquid suspensions for oral administration.  Compounding pharmacies may be able to produce palatable tablets and liquids for pets.  Veterinarians sometimes administer antihistamines by injection.

Side Effects

Pets rarely suffer side effects from antihistamines. However, sedation, agitation, and gastrointestinal upset (loss of appetite, diarrhea, or vomiting) are possible.

Antihistamines may exacerbate certain forms of glaucoma (an eye disorder). As well, they must be used cautiously in animals with heart disease (cats, dogs) and cats with hyperthyroidism.  Some antihistamines have been linked to cognitive decline (dementia) in humans.  No such link has yet been made in animals.

Contact your veterinarian immediately if you suspect your pet is suffering side effects from antihistamines.


Owners should monitor pets for side effects and for resolution of symptoms such as itching or anxiety.

Pets that receive long-term medications may benefit from periodic blood and urine tests to monitor the function of the liver and kidneys.

Copyright © Eric Barchas, DVM All rights reserved.
The contents of this page are provided for general informational purposes only. Under no circumstances should this page be substituted for professional consultation with a veterinarian.