By Rania Gollakner, BS, DVM, MPH

What is diphenhydramine?

Diphenhydramine (brand name: Benadryl®, Vetadryl®, Banophen®, Genahist®, Diphenhist®, Unisom®, Sominex ®) is an antihistamine used in cats, dogs, ferrets and other small mammals, birds, horses, and cattle to treat allergic reactions, motion sickness, and vomiting. It is also used as a mild sedative, and is one of the therapies for mast cell tumors.

Its use in small and large animals to treat allergic reactions and other conditions is ‘off label’ or ‘extra label’. Many drugs are commonly prescribed for off label use in veterinary medicine. In these instances, follow your veterinarian’s directions and cautions very carefully as their direction may be significantly different from those on the label.

How is diphenhydramine given?

Diphenhydramine is given by mouth in the form of a tablet, capsule, oral liquid, or liquid injection. The oral formulations can be given with or without food. If your pet vomits after dosing on an empty stomach, give future doses with food or a treat. The liquid has a bad taste to cats, and administration by mouth may be difficult. The injection is typically given in the hospital, either in the vein, muscle, or under the skin.

This medication will take effect quickly, in about 1 to 2 hours, and improvement in clinical signs should follow.

What if I miss giving my pet the medication?

If you miss a dose, give it when you remember, but if it is close to the time for the next dose, skip the dose you missed and give it at the next scheduled time, and return to the regular dosing schedule. Never give your pet two doses at once or give extra doses.

Are there any potential side effects?

The most common side effect is lethargy, dry mouth, and urinary retention. Vomiting, diarrhea, and lack of appetite are also possible. Excitement rather than lethargy can occur occasionally, especially in cats. Diphenhydramine can alter the results of skin allergy testing; discontinue this medication at least 2 weeks before testing.

This short-acting medication should stop working within 24 hours, although effects can be longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

Diphenhydramine should not be used in patients that are allergic to it or similar antihistamines. Do not use in pediatric or neonatal pets. Diphenhydramine should be used cautiously in pets with glaucoma, prostate enlargement, intestinal or bladder obstruction, COPD, elevated thyroid hormone, heart disease, or high blood pressure. Use with caution in pets that are pregnant or lactating, geriatric, or are working dogs.

Are there any drug interactions I should be aware of?

The following medications should be used with caution when given with diphenhydramine: anticholinergic drugs, tricyclic antidepressants, and CNS depressant agents such as sedatives, anesthetics, and pain medications.

Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking.

Is there any monitoring that needs to be done with this medication?

There is no specific monitoring that needs to be done while your pet is taking this medication. Your veterinarian may monitor your pet to be sure that the medication is working. Monitor your pet at home for adverse effects.

How do I store diphenhydramine?

All formulations of diphenhydramine should be stored at room temperature between 15°C and 30°C (59°F to 86°F) and protected from light. Oral liquids should be protected from freezing.

What should I do in case of emergency?

If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility.

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