By Wendy C. Brooks, DVM, DipABVP
Educational Director, VeterinaryPartner.com
(Hydroxyzine hydrochloride, Hydroxyzine pamoate)
Brand Name: Atarax, Masmoran, Vistaril
Available in 10 mg, 25 mg, 50 mg & 100 mg tablets(hydroxyzine hydrochloride); 25 mg, 50 mg, 100 mg capsules (hydroxyzine pamoate); oral suspension
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:
In the past, antihistamines have been used more widely to manage itch and allergy but recent work has not been able to demonstrate a clear positive effect from antihistamines directly, either for acute allergy flare ups or in the long-term management of itch. They are probably best used long term as a preventive of allergic itch rather than in an acute itchy situation as they cannot block H1 receptors that are already actively occupied by histamine but can prevent histamine binding before the fact. Long-term antihistamine use can also be helpful in reducing the need for stronger medications such as corticosteroids (prednisolone, etc.).
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 of any type could be used to help control mast cell tumor symptoms.
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.
There are many classes of antihistamines. If one antihistamine is ineffective, it is possible that another might yet be effective as long as it is in a different class from the original antihistamine. Hydroxyzine is of the piperazine class. Antihistamines are also famous for inducing tolerance, which means that after a long enough period, the medication will no longer work. In this situation, an antihistamine of a different class could be used. If a medication is withdrawn after tolerance has developed, if one waits a long enough period, tolerance will likely fade and the medication may again be effective.
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 dry mouth may be observed drinking more water. Because of the difficulty with the 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 treating 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 (Zyrtec) 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.
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