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

Brand Name: Benadryl

Available in 25 mg & 50 mg capsules, 12.5 mg chewable tablets, 50 mg tablets, oral elixir and syrup

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.

How this Medication is Used

Diphenhydramine has several important effects and thus several uses. Most obviously, diphenhydramine 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

Diphenhydramine is frequently included in antihistamine trials for allergic skin disease. It is not one of the more effective antihistamines in this regard but its availability and inexpensive cost make it worth trying in many cases.

See more information on the management of itchy skin.

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.
Diphenhydramine has a strong anti-nausea side effect that makes it helpful in treating motion sickness.

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

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.

Interactions with other Medications

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.

Diphenhydramine should not be used with additional tranquilizing medications.

In animals that experience a hyperactivity reaction while taking metoclopramide (Reglan), this effect can be reversed with a dose of diphenhydramine.

Concerns and Cautions

Be careful of oral liquids containing alcohol. These formulations should not be used in pets.

It is important to realize that “Benadryl” is a brand name. It is easy to try to save money and try to buy over-the-counter diphenhydramine, and end up with an inappropriate product. To emphasize this point, see the products below. It seems like the one on the left would contain diphenhydramine, but in fact it contains a combination of diphenhydramine and other medications, including a dose of acetaminophen which could be lethal to a pet.

Never buy over-the-counter medication for your pet without knowing exactly what you are supposed to get and never medicate  your pet without  your vet’s guidance.

This product contains a combination of medications with substantial potential for harm if given to a pet.

This product contains only diphenhydramine, with no additional medications added.

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

Date Published: 1/1/2001
Date Reviewed/Revised: 03/26/2008