Antihistamines for Dogs — Everything You Need to Know

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Author: Jacob Kay

You’re having a wonderful day with your dog, playing fetch in the park or snuggling on the couch. Suddenly, you see your dog itching fiercely, sneezing violently or swelling up like a balloon. You recognise that your dog is allergic to something and wonder what you can do to help them.

You have nothing to help but just antihistamines. But wait, are antihistamines safe for dogs? Let us explore some benefits as well as side effects of antihistamines on dogs.

Antihistamines for Dogs

Antihistamines for Dogs

Antihistamines can treat allergies like redness, itchiness or swelling. Antihistamines work by blocking histamine receptors. Histamine is a molecule that the body releases in response to an allergen. They are inexpensive, readily available as over-the-counter medications, and generally free of side effects.

As a result of Histamine release, unlike humans, dogs will typically have redness and itchiness of their skin as their main symptoms. Antihistamine can be used to treat the symptoms of mild allergies, to soothe a bee or wasp sting and to get relief from itchiness.

when should antihistamines be used?

Bee/Wasp Sting

When your dog is stung by a bee or a wasp, the primary thing to do is take out the sting from the body of your dog. You can clean the area with a wet cloth or hold the area under running water for your dog to get relief.

You can also apply ice to reduce the swelling and burning sensations. At this stage, you can give your dog an antihistamine for dogs uk to ease the symptoms. However, you should also as your vet before doing so.


If your dog is allergic, your vet may suggest giving antihistamines to your dog. The allergy might be caused due to some food habit, pollen or any medication used for your dog. Antihistamine can be used as part of treatment to soothe the symptoms of allergies.[1]

It is crucial to find out the underlying cause of the allergy by observing your dog’s diet and medication. This will help curb the root cause of itchiness and help cure allergies.

What Is the Dosage Schedule for Antihistamines?

The typical dosage schedule for antihistamines in dogs depends on the type of antihistamines, the weight of the dog, and the severity of the allergies. Here are some examples of common antihistamines and their dosages for dogs:

  • Diphenhydramine (Benadryl®): 2–4 mg/kg, every 8–12 hours as needed. This is equivalent to 1–2 mg per pound of body weight.
  • Hydroxyzine: 0.5–2 mg/kg, every 6–8 hours as needed. This is equivalent to 0.23–0.91 mg per pound of body weight.
  • Chlorpheniramine: For dogs less than 20 kg (44 pounds), 4 mg, every 8 hours as needed. For dogs more than 20 kg (44 pounds), 8 mg, every 8 hours as needed.
  • Cetirizine (Zyrtec®): For dogs, 1 mg/kg or 10–20 mg per dog, every 12–24 hours as needed. This is equivalent to 0.45 mg per pound of body weight for dogs.
  • Fexofenadine (Allegra®): For dogs, 2–5 mg/kg, PO, every 12–24 hours as needed. This is equivalent to 0.91–2.27 mg per pound of body weight for dogs and cats.

These are general guidelines and may vary depending on the individual dog’s condition and response to the medication. Always consult your veterinarian before giving your dog any antihistamines and follow their instructions carefully.

What Are the Side Effects of Antihistamines in Dogs?

Antihistamines are medications that can help reduce the symptoms of allergies in dogs such as itching, sneezing and swelling. However, since they are chemicals, they can also have some side effects that you should be aware of before giving them to your dog.

side effects on antihistamines

Some of the possible side effects of antihistamines in dogs are:

  • Drowsiness and lethargy. Antihistamines might cause your dog to become lethargic and less active, affecting their daily routine and quality of life. This can also be hazardous if your dog is required to be awake or execute specific responsibilities. This is most prevalent with first-generation antihistamines like diphenhydramine.
  • Dry mouth and blurred vision. Antihistamines might lower your dog’s saliva and tear production, resulting in dry mouth and eyes. This can cause discomfort as well as an increase in the risk of infections and dental disorders.
  • Vomiting and diarrhoea. Antihistamines can cause nausea, vomiting, and diarrhoea in your dog. This can result in dehydration and electrolyte imbalance, both of which can be fatal if not treated swiftly.
  • Excitability and nervousness. Some dogs may react to antihistamines differently, becoming more nervous and worried rather than calm and relaxed. This might result in behavioural issues and stress for both your dog and you.

When Should You Not Use Antihistamines on Your Dog?

As a paw parent, you should be cautious when using antihistamines on your pet. Firstly, it is important for you to consult your local vet before experimenting with any medicine on your Fido. You should never use antihistamines if your dog has glaucoma, in which the pressure within the eyes increases and it leads to vision loss.[2]

Dogs suffering from heart or liver disease should also avoid the usage of antihistamines. Moreover, this medicine can also affect the development of fetuses and thus, you should not give it to pregnant dogs. There may be conditions depending on your dog’s individual health status and history. Therefore, it is always advisable to check with your vet before giving any medication to your dog.

According to Malcolm Weir and Lynn Buzhardt at VCA Hospitals, “Antihistamines are usually safe but can make some dogs drowsy and others hyperactive. OTC antihistamine preparations may contain other ingredients such as decongestants that are not safe for dogs.

You should not give antihistamines to dogs with conditions like glaucoma, heart and liver disease, pregnant dogs, or vaginitis in dogs or other vaginal infections.

What Are the Commonly Used Antihistamines for Dogs?

The most commonly used antihistamines for dogs include:

  • Diphenhydramine (Benadryl®)
  • Loratadine (Claritin®)
  • Cetirizine (Zyrtec® or Reactine®)
  • Chlorpheniramine (Chlortrimeton®)
  • Hydroxyzine (Vistaril®)

Claritin® can be used however, veterinarians normally do not advocate it since some types of Claritin® contain decongestants such as pseudoephedrine, which is extremely harmful to dogs. Cetirizine (Zyrtec® or Reactine®) is a popular over-the-counter antihistamine that veterinarians suggest.


What antihistamine can I give my dog?

Antihistamines with the active ingredients of diphenhydramine, chlorpheniramine, cetirizine, clemastine or loratadine are usually safe for most healthy adult dogs. Hydroxyzine is another dog-safe antihistamine and one of the most effective available, but it’s only available on prescription.

What is the safest antihistamine to take?

Claritin and Zyrtec are popular over-the-counter antihistamines. Doctors consider them safe and effective treatments for minor allergies. Both are second-generation antihistamines. These cause less drowsiness than first-generation antihistamines.

Is too much antihistamine bad for dogs?

In living animals, hyperactivity, depression, hypersalivation, tachypnea, and tachycardia are the most common signs reported with ethanolamine-based antihistamines, usually occurring within 1 h of exposure. With overdose, dogs may also exhibit mydriasis, dry mucous membranes, disorientation, and fever.


Antihistamines can be a useful alternative for dogs with mild to moderate allergies since they can relieve some of the discomfort and irritation caused by allergens. Antihistamines, on the other hand, are not a universal remedy and may not be acceptable or effective for every dog.

As a result, it is critical to consult your veterinarian before administering antihistamines to your dog and to carefully follow their advice. You should also keep an eye on your dog for any signs of adverse behavior or overdosing. You may help your dog have a happier and healthier life by doing so.

While antihistamines can provide relief for allergy symptoms, be aware of other conditions like kennel cough which can be mistaken for allergies but may be more serious if left untreated. For more details check out our article on "can kennel cough kill a dog".


  1. Purdue Veterinary Medicine. (n.d.). Pet health tips. Purdue University College of Veterinary Medicine.
  2. Glaucoma in dogs | VCA Animal Hospitals. (n.d.). VCA Hospitals.
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Jacob Kay
Jacob Kay is a Veterinary Advisor and Editor at WWD. He’s also a dog lover and has two pet dogs of his own. He has extensive knowledge in the field of veterinary medicine and is always happy to share his insights with others.

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