Diphenhydramine (Benadryl®)
By: Dr. Dawn Ruben

Overview

Histamine is a chemical that is released in the body in response to inflammation or allergy. This chemical travels throughout the body searching for specific histamine receptors (targets on cells). Once attached to the receptors, histamine will cause swelling, itchiness and other symptoms associated with an allergic response.

There are two types of histamine receptors: H1 and H2. H1 receptors affect small blood vessels and smooth muscles. When histamine attaches to the H1 receptors, the small blood vessels dilate and fluid begins to leak out. This results in tissue swelling and itchiness. In addition, the smooth muscles lining the small airways constrict, causing tightness and some breathing difficulty. H2 receptors affect heart rate and stomach acid secretions. When histamine attaches to H2 receptors, the heart rate increases and stomach acid secretions are increased, potentially increasing the risk of developing ulcers.

Drugs that block the effects of histamine are called antihistamines. There are a number of drugs demonstrating antihistamine effects; some are useful in allergies, others for preventing excessive stomach acid. The effects of the antihistamine depend on whether it binds with the H1 receptors or H2 receptors. There are few drugs that affect both types of receptors.

Diphenhydramine is one type of antihistamine that inhibits the action of histamine, particularly its effect on H1 receptors. This results in a reduction or prevention of swelling and itchiness. Diphenhydramine has little to no effect on heart rate or stomach acid secretions.

Diphenhydramine is available over the counter but should not be administered unless under the supervision and guidance of a veterinarian.

This drug is not approved for use in animals by the Food and Drug Administration but it is prescribed legally by veterinarians as an extra-label drug.

Brand Names and Other Names

This drug is registered for use in humans only.

Human formulations: Benadryl® (Parke-Davis) and various generic preparations

Veterinary formulations: None

Uses of Diphenhydramine

Diphenhydramine is used primarily to treat allergic symptoms, itchy skin and allergic reactions such as that caused by a drug or an insect bite.

It is also used to treat motion sickness and vomiting because of effects on the brain and nervous system.

Precautions and Side Effects

While generally safe and effective when prescribed by a veterinarian, diphenhydramine can cause side effects in some animals.

Diphenhydramine should not be used in animals with known hypersensitivity or allergy to the drug.

Animals with glaucoma, lung disease, heart disease, an overactive thyroid, high blood pressure and prostate enlargement should not use diphenhydramine.

Diphenhydramine may interact with other medications. Consult with your veterinarian to determine if other drugs your pet is receiving could interact with diphenhydramine. Such drugs include epinephrine, tranquilizers, heparin and barbiturates.

The most common adverse effects of diphenhydramine are sedation, lethargy, vomiting, diarrhea and lack of appetite.

In cats, oral diphenhydramine is bitter and can cause profuse, yet temporary, drooling.

How Diphenhydramine Is Supplied

Diphenhydramine is available in 12.5 mg, 25 mg and 50 mg tablets.

It is also available as a 12.5 mg/ml suspension.

Injectable diphenhydramine is available in 10 mg/ml and 50 mg/ml concentrations.

Dosing Information

Medication should never be administered without first consulting your veterinarian.

Diphenhydramine is dosed in dogs at 0.5 to 2 mg per pound (1 to 4 mg/kg) orally two to three times daily or 0.5 to 1 mg per pound (1 to 2 mg/kg) IM, IV or SC twice daily.

In cats, diphenhydramine is dosed at 0.25 to 0.5 mg per pound (0.5 to 1 mg/kg) PO every 12 hours but it has a bitter taste. It is also dosed at 0.5 to 1 mg per pound (1 to 2 mg/kg) IM every 12 hours.

The duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your pet feels better, the entire treatment plan should be completed to prevent relapse.

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Diphenhydramine (Benadryl®)
By: Dr. Dawn Ruben

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