THE PET PHARMACY By Wendy C. Brooks, DVM, DipABVP Educational Director, VeterinaryPartner.com

(Hydroxyzine hydrochloride, Hydroxyzine pamoate)

Brand Name: Atarax

Available in 10 mg, 25 mg, 50 mg & 100 mg tablets(hydroxyzine hydrochloride) and 25 mg, 50 mg, 100 mg capsules (hydroxyzine pamoate) as well as oral suspension

Background

Histamine is an inflammatory biochemical that causes skin redness, swelling, pain, increased heart rate, and blood pressure drop 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 antihistamine therapy are not as reliable in pets, particularly dogs, when it comes to itchiness. There are, however, numerous uses for antihistamines, including hydroxyzine, in veterinary medicine.

How this Medication is Used

Hydroxyzine has several important effects and thus several uses. Most obviously, hydroxyzine is an antihistamine and it is used for acute inflammatory and allergic conditions such as:

• Snake bites
• Vaccination reactions
• Blood transfusion reactions
• Bee stings and insect bites
• The management of itchy skin

Hydroxyzine is frequently included in antihistamine trials for allergic skin disease. In such trials, several antihistamines are used sequentially in hoping to find one that was especially effective. Antihistamines in general are more effective in itchy cats than itchy dogs although many dogs achieve relief with this medication. The enteric coating and small sized tablet make it a common first choice in an antihistamine trial.

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, although hydroxyzine could also be used.

Hydroxyzine has an anti-nausea side effect that makes it helpful in treating motion sickness.

Hydroxyzine causes drowsiness in animals just as it does in people and can be used as a mild tranquilizer.

Hydroxyzine is a member of the piperazine class of antihistamines. When different antihistamines are being tried and one appears ineffective, it is felt that the subsequent choice should be of a different class to avoid the likelihood of another failure.

Side Effects

With so many possible uses of this medication, it is difficult to separate out a side effect from a primary effect. Drowsiness is generally regarded as an undesirable side effect.

At doses higher than the recommended dose, human patients complain of dry mouth and experience difficulty with urination. Animal patients experiencing a dry mouth may be observed drinking more water. Because of the difficulty with urination side effect, this medication and others with similar side effects should be avoided in patients with bladder neck tumors (such as the transitional cell carcinoma) or with prostate enlargement.

Fine tremors and even seizures have been reported in dogs taking this medication.

Interactions with other Drugs

In the treatment of allergic skin disease, antihistamines are felt to synergize with omega 3 fatty acid supplements and, as a general rule for this condition, it is best to use these medications together.

Hydroxyzine should not be used with additional tranquilizing medications.

A newer antihistamine called cetirizine has become available. This medication represents an active form of hydroxyzine that does not cross the blood-brain barrier. This means that cetirizine would be helpful for a patient who has responded well to hydroxyzine but had an unacceptable level of drowsiness.

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 CAN CAUSE BIRTH DEFECTS IN PREGNANCY. IT SHOULD NOT BE USED IN PREGNANT PETS.

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

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

Date Published: 1/1/2001
Date Reviewed/Revised: 03/21/2008