A guide to antihistamines for dogs

Just as we humans can develop allergies to pollen, dust and all manner of things, so too can dogs. In people (and dogs!) with allergies, the immune system treats harmless allergens, such as pollen, as a threat. It reacts by producing antibodies to try and fight off this perceived threat.

Part of it’s response to allergens is to release histamine into the bloodstream. It’s this release of histamine that causes many allergy symptoms.

And antihistamines, as the name suggests, are drugs that block the effects of histamine. This is what makes them effective in treating allergies.

Diphenhdyramine is the most common antihistamine. It’s found in both prescription and over the counter antihistamines.

Other antihistamines include clemastine, cetirazine, chlorpheniramine, hydroxyzine, and cyproheptadine. The table below gives more details about all these antithistamines.

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Antihistamines for dogs

You can use antihistamines to treat dog allergies just as well as human allergies. It’s important to note that they don’t cure the allergies, they just ease the symptoms.

And, as mentioned earlier, there are several different types of antihistamines. Some of these are more effective in dogs than others.

It’s always best to consult with your vet to help find the best antihistamine for your dog. The most effective antihistamine can vary according to the dog breed. It can also depend on your dog’s age, weight and medical history. That said, the most common over the counter antihistamine for dogs is Benadryl.

You may have to spend several weeks trying out three or four different antihistamines. It’s a good idea to do this when your dog’s symptoms are at their worst. That way, it’s easier to know if the antihistamine is working for your dog or not.

You shouldn’t give some antihistamines to dogs with certain medical conditions (see the table above). This is another reason why it’s important to talk to your vet first.

A guide to antihistamines for dogs

Side effects

Antihistamines are generally safe for dogs. They can, though, have some side effects in up to a quarter of dogs. These are most often mild and disappear once you stop treating your dog.

Some side effects of antihistamines you may see include:

  • Drowsiness.
  • Lethargy.
  • Thirst.
  • Lack of appetite.
  • Hyperactivity.
  • Constipation.
  • Diarrhea.
  • Vomiting.

The different types of antihistamines can have different side effects. Consult with your vet for more details.

Antihistamine dosage for dogs

The correct antihistamine dosage for dogs depends on the particular antihistamine. It also depends on the size of the dog.

In the tables below are some typical recommended dosages for various antihistamines. Once more, I recommend you should always consult your vet first. They will help you to determine the best antihistamine and dosage for your dog.

Cetirazine dosage

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The cetirazine dosage assumes 5mg tablets.

Chlorpheniranime dosage

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The chlorpheniranime dosage assumes 4mg tablets.

Clemastine dosage

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The clemastine dosage assumes 1.34mg tablets.

Cyproheptadine dosage

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Cyproheptadine tablets are usually 2mg or 4mg. The above dosage assumes 4mg tablets.

Diphenhdyramine dosage

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Diphenhdyramine tablets are usually 25mg or 50mg.

Hydroxyzine dosage

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Hydroxyzine tablets are usually 10mg, 25mg or 50mg.


Antihistamines can be a safe and effective allergy treatment for dogs. There are various types of antihistamines. Some of them are available over the counter and others are prescription only.

You should always talk to your vet before giving your dog medication. Your vet will be able to help you to determine the best antihistamine for your dog. They’ll also be able to tell you the correct dosage.

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