Antihistamines for Dogs: Zyrtec, Telfast and Phenergan

It's 10pm and your dog has been scratching for an hour. You have a box of Zyrtec in your medicine cupboard. Vets do use antihistamines in dogs, but there are some pitfalls to be aware of. This guide will show you which antihistamines are which, how well antihistamines for dogs work for itch, and what ingredient to watch out for on your box.

This is general information only and should not replace advice from your vet who knows your dog.

Can dogs have antihistamines?

Vets do sometimes use antihistamines in dogs. Cetirizine, fexofenadine and others are included in the list of medications used for itchy allergic dogs (Merck Veterinary Manual). They are commonly used for hives.

However, they are human medications, and the safety of most over-the-counter human medications hasn't been worked out in animals (MSD Veterinary Manual), so it is important to speak to your vet before giving any of these to your pet. The correct medication and dose must be determined based on your dog's size, age, health and any medications they may currently be taking. Please don't guess.

What's in the antihistamines you have at home

Most antihistamines in Australia are sold under brand names. Below are the active ingredients in common antihistamines:

• Zyrtec: cetirizine
• Claratyne: loratadine
• Telfast: fexofenadine
• Phenergan: promethazine, which is known for causing drowsiness
• Polaramine: dexchlorpheniramine

What about Benadryl?

A lot of the advice online about Benadryl for dogs is written for products sold overseas. In Australia there are a variety of Benadryl products for coughs and colds. Some contain diphenhydramine and other ingredients. Some don't even contain any antihistamines and some contain a decongestant called phenylephrine. The product you get from a chemist in Australia may not be the same as described on an overseas website.

Do antihistamines actually work for itchy dogs?

This may surprise you. Current research doesn't support the usage of antihistamines to help reduce itch in dogs (Merck Veterinary Manual). Effectiveness can vary between individual dogs.

According to international guidelines developed by the International Committee on Allergic Diseases of Animals, oral antihistamines may provide a small and limited benefit in some dogs. They are most effective when used daily to help prevent itch, rather than once a flare-up has started.

Antihistamines alone are often not effective for dogs with severe itching. The most common causes of itching are parasites, infections and allergies (Merck Veterinary Manual), and each needs a different treatment. Your vet will often check for fleas as a first step, and the guidelines recommend year-round flea control for allergic dogs.

What your vet may use instead

If your dog's itch is due to allergies, your vet may prescribe a stronger medication designed specifically for dogs. These include:

• Oclacitinib (Apoquel), a tablet registered in Australia to help reduce itch caused by allergic skin disease in dogs 12 months of age and older
• Lokivetmab, an antibody treatment given by your vet for allergic skin disease, which usually starts working within one to three days
• Steroids such as prednisolone for severe flare-ups
• Treatment for skin or ear infections, which often make itch worse

The decongestant danger

This is the warning that matters most. Some antihistamines contain a decongestant. Telfast Decongestant and Claratyne-D with Decongestant both contain pseudoephedrine in Australia. Loratadine and fexofenadine are commonly combined with pseudoephedrine, which can cause more serious poisoning (Merck Veterinary Manual).

A pseudoephedrine overdose can cause dilated pupils, a rapid heart rate and high blood pressure in pets. Symptoms can last one to four days. An overdose of antihistamines alone can also cause issues such as your dog being very flat or hyperactive, drooling, rapid breathing and shaking.

Liquid medications can be a pitfall. Some sugar-free syrups contain xylitol, which is toxic to dogs.

If your dog has eaten any human medications or you have given them one and they are behaving strangely, call your vet or the Animal Poisons Helpline immediately on 1300 869 738. Have the packet with you.

When hives or swelling is an emergency

Allergic reactions in dogs are often caused by insect bites or stings. You may notice small raised bumps on their skin or swelling of the face (especially lips, muzzle and around the eyes). Hives and facial swelling are usually not life-threatening, and vets often treat them with antihistamines (MSD Veterinary Manual).

Hives can be a first sign of anaphylaxis. This is a severe reaction and can be life-threatening. Signs in dogs include sudden vomiting or diarrhoea, drooling, pale gums, cold legs, shock and seizures. If you notice any of these signs, don't wait to see if an antihistamine helps. Seek immediate care from your closest emergency vet.

Frequently asked questions

Can I give my dog Zyrtec?

Cetirizine is one of the antihistamines used by vets in dogs. Speak to your vet to confirm this is suitable for your dog. They will calculate a safe dose and want to know if the cause of the itching is something that antihistamines will not help with.

Is Phenergan safe for dogs?

Phenergan contains promethazine, a sedating antihistamine that's known to cause drowsiness. Don't give it to your dog unless your vet says it is suitable.

How much antihistamine can I give my dog?

There's no single safe dose that fits every dog. The dose depends on the medication, weight and health of your dog. Please speak to your vet.

Why is my dog still itchy on antihistamines?

Antihistamines only provide a small benefit for some dogs. They will not help with fleas, mites or skin infections. See your vet to determine the cause of the itching.

Need help with an itchy dog?

Constant scratching, licking and chewing can make your dog very unhappy and create an unsettled household. We can come to you to check your dog's skin and ears without the stress of a car trip and a waiting room. Our Pawssum vets have helped more than 380,000 pets across Australia and are rated 4.6 from 2,805 Google reviews.

We're open 6am to 11pm, 7 days a week. Book a home visit online or call 0480 030 580.