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By Wendy C. Brooks, DVM, DipABVP
Educational Director,

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Cetirizine Hydrochloride (Zyrtec)

Brand Name: Zyrtec

Available in 5 & 10 mg tablets, chewable and non-chewable, as well as oral syrup


Histamine is an inflammatory biochemical that causes skin redness, swelling, pain, increased heart rate, and a drop in blood pressure when it binds to one of many H1 receptors throughout the body. Histamine is an important mediator of allergy in humans, hence a spectacular array of different antihistamines has proliferated. Histamine, perhaps unfortunately, is not as important a mediator of inflammation in pets, which means results of using antihistamines are not as reliable in pets, particularly dogs, when it comes to itchiness.

That said, antihistamines are still widely used to assist in controlling itching in pets as well as people. Also, in pets and people the drowsiness side effect of antihistamines has been undesirable and there has been a drive for antihistamines that do not cause drowsiness. Cetirazine is the result of such efforts. It turns out that when hydroxyzine, a popular antihistamine, is metabolized by the body, cetirazine is one of the by-products. Cetirazine is an antihistamine in its own right but does not cross the blood-brain barrier and thus should not produce the drowsiness side effect.

How This Medication Is Used

Cetirizine is an antihistamine and it is used for acute inflammatory and allergic conditions such as:

In the past, antihistamines have been used more widely to manage itching and allergies but recent work has not been able to demonstrate a clear positive effect directly from antihistamines, either for acute allergy flare ups or in the long-term management of itching. Antihistamines seem best used concurrently with corticosteroids (such as prednisone) in a more long-term setting so as to enable a lower corticosteroid dose to be just as effective as a higher one. 

Mast cell tumors are tumors involving cells that contain granules of histamine. Patients with mast cell tumors experience chronic inflammatory symptoms due to circulating histamine. Antihistamines such as diphenhydramine may be helpful given long term though cetirizine could also be used.

Cetirizine is a member of the piperazine class of antihistamines. When different antihistamines are being tried and one appears ineffective, the next choice should be of a different class to avoid the likelihood of another failure.

Side Effects

Because cetirizine does not cross into the nervous system, side effects common to other antihistamines, such as dry mouth or heart rate elevation, are rare.

Drowsiness is still reported in 13% of people who take cetirizine.

The biggest problem with long-term antihistamine use is developing tolerance. This means that the body may eventually fail to respond to the medication as well as it did initially. Happily, tolerance is not felt to occur with cetirizine use.

Interactions With Other Drugs

Cetirizine should not be used with tranquilizing medications. Even though cetirizine was designed not to be sedating on its own, combining it with another medication with sedating properties will yield an additive effect.

Concerns And Cautions

When using an antihistamine to prevent an allergic reaction (such as a vaccine reaction), the antihistamine works best when given prior to the allergen.

This medication will interfere with allergic skin testing. Check with your veterinary dermatologist regarding how far in advance this medication should be withheld.

There is a formulation of cetirizine called Zyrtec-D that combines cetirizine with pseudoephedrine, a decongestant. This product (as well as similarly formulated generics) are not appropriate for animal use as the pseudoephedrine has toxicity potential.

It is our policy not to give dosing information over the Internet.


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