Diphenhydramine (Benadryl®) for Dogs and Cats

A number of pink Benadryl pills in a blister pack.A number of pink Benadryl pills in a blister pack.
A number of pink Benadryl pills in a blister pack.A number of pink Benadryl pills in a blister pack.

Table of Contents:

  1. Overview of Benadryl
  2. Uses of Diphenhydramine for Dogs and Cats
  3. Precautions and Side Effects
  4. Benadryl Dosing for Dogs and Cats
  5. Drug Library for Pet Owners

Diphenhydramine, commonly known as Benadryl®, is one of the most popular human and pet over-the-counter medications. It is categorized as an antihistamine.  It is routinely prescribed in dogs and occasionally used in cats.

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.

Overview of Benadryl

This drug is registered for use in humans only.

  • Human formulations: Benadryl® (Parke-Davis) and various generic preparations
  • Veterinary formulations: None

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.

Benadryl is available in the following formulations:

  • 12.5 mg, 25 mg, and 50 mg tablets
  • 12.5 mg/5 ml suspension
  • 10 mg/ml and 50 mg/ml concentrations of injectable diphenhydramine

Uses of Diphenhydramine for Dogs and Cats

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. Common adverse side effects in both dogs and cats include:

  • Difficulty urinating
  • Dry mouth
  • Excessive drooling
  • Lethargy
  • Rapid breathing and heart rate

Rarer side effects include:

  • Diarrhea
  • Decreased or increased appetite
  • Vomiting

These symptoms tend to present themselves within an hour of ingesting Benadryl. Signs of a severe overdose include agitation, an especially rapid heart rate, agitation, and seizures. Pets owners who observe these symptoms should contact their veterinarian or a pet poisoning hotline 

Unless recommended by a veterinarian, Benadryl should not be administered to pregnant pets, pets with known allergies to the drug, or animals with any of the following conditions:

  • Cardiovascular disease
  • Enlarged prostate
  • Glaucoma
  • High blood pressure
  • Low blood pressure
  • Lung disease
  • Overactive thyroid glands
  • Seizure disorders

Diphenhydramine may interact with other medications. Consult with your veterinarian to determine if other drugs including epinephrine, tranquilizers, heparin, and barbiturates could cause side effects for your pet.

Always monitor your pet closely after administering Benadryl to identify potential warning signs early.

Benadryl Dosing for Dogs and Cats

The Merck Veterinary Manual advises pet owners that the proper dosage of Benadryl for dogs is typically between two to four milligrams per kilogram of body weight, administered two to three times daily. In cats, a typical dose of Benadryl for cats is one milligram per pound of body weight, administered every twelve hours. Keep in mind that cats may resist due to the drug’s bitter taste.

The duration of administration depends on the condition being treated, your pet’s response to the medication, and the potential development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian.

Pet owners are warned against administering time-release capsules to their pets. The American Kennel Club (AKC) notes that pets metabolize them differently than humans and that they could break open upon ingestion.

Remember that medication should never be administered without first consulting your veterinarian.

Drug Library for Pet Owners

Need more information about safely administering prescription medication to your pet? While your veterinarian should always be your first resource for questions related to your pet’s health, PetPlace’s Drug Library features a wealth of valuable information as well.

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