Benadryl for Dogs: Is Diphenhydramine Safe for Dogs?

Diphenhydramine, commonly known as Benadryl®, is one of the most popular human and pet over-the-counter medications. It is categorized as an antihistamine and has been used commercially since the mid-1940s. Benadryl is one of 15 human over-the-counter drugs safe for dogs.

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.

Here are some additional facts about histamine and antihistamines:

• 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.

What Is Benadryl?

This drug is registered for use in humans only. There are many uses of diphenhydramine in humans, including as a treatment for allergies, trouble sleeping (insomnia), nausea, tremors, and for symptoms related to cold viruses.

• Human formulations: Benadryl® (Parke-Davis), Genahist, Sominex, Unisom, and various generic preparations
• Veterinary formulations: Calmatrol (Goldman Pharmaceutical), Vetadryl (Pegasus), and various generics.

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 (FDA), but it is prescribed legally by veterinarians as an extra-label drug.

Benadryl is available in the following formulations:

• 10 mg, 12.5 mg, 25 mg, 30 mg, and 50 mg tablets
• 12.5 mg/5 ml suspension
• 2 mg/ml, 10 mg/ml, and 50 mg/ml concentrations of injectable diphenhydramine
• Shampoo and topical liquid formulations are also available and are marketed to treat rashes, though these are not commonly used for dogs.

Uses of Diphenhydramine for Dogs

Diphenhydramine is used primarily to treat chronic allergic symptoms, such as itchy skin, acute allergic reactions that cause hives and skin swelling, vaccine or drug reactions, snake bites, insect stings, or from unknown causes.

Here are other uses of Benadryl for dogs:

• Benadryl is commonly given prior to vaccines in dogs with a history of vaccine reactions.
• It is also given to dogs that have an adverse reaction to a blood transfusion.
• It is used to treat motion sickness and vomiting because of effects on the brain and nervous system (chemoreceptor trigger zone). However, there are other drugs for motion sickness that are more effective, such as maropitant (Cerenia®).
• Diphenhydramine is used before surgery in dogs with mast cell tumors.
• Benadryl can be used in dogs with vestibular syndrome. This is a disorder that affects the balance and equilibrium.

Although diphenhydramine can be safe and effective in dogs, there are other medications for chronic allergies that have proven more effective. Food allergies or allergies due to environmental exposures are common in dogs and can cause itching, hair loss, and skin infections. Some studies suggest that antihistamines are only effective in controlling symptoms of chronic allergies in 25% of dogs. Diphenhydramine can be used in combination with other drugs such as steroids (e.g., Prednisone). For more information about chronic allergies, please read about atopic dermatitis and food allergies in dogs. Common medications successfully used for dog allergies include Oclacitinib (Apoquel®) and Cytopoint®. For itching and scratching home care tips, click here.

Poor response to treatment should prompt a call to the prescribing veterinarian. Other drugs besides antihistamines may be more effective to treat some conditions, or diphenhydramine may need to be used in conjunction with other medications.

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 dogs include:

• Difficulty urinating caused by urinary retention
• Dry mouth
• Excessive drooling
• Lethargy and sedation
• Rapid breathing and heart rate
• Wobbly or unsteady walking
• Agitation
• Dilated pupils

Rarer side effects include:

• Diarrhea
• Decreased or increased appetite
• Vomiting

These symptoms tend to present themselves within an hour of ingesting Benadryl. Unless recommended by a veterinarian, Benadryl should not be administered to pregnant or nursing pets, pets with known allergies to the drug, or animals with any of the following conditions:

• Cardiovascular disease (heart failure)
• Enlarged prostate
• Glaucoma
• Hypertension (high blood pressure)
• Low blood pressure
• Lung disease (especially chronic obstructive pulmonary disease [COPD] or asthma)
• Overactive thyroid glands
• Seizure disorders
• Bladder neck obstruction

Drug Interactions with Benadryl

Always monitor your pet closely after administering Benadryl to identify potential warning signs of an adverse drug reaction. Diphenhydramine may interact with other medications. Consult with your veterinarian to determine if drugs including epinephrine, tranquilizers, antifungals, heparin, barbiturates, and some antibiotics could cause side effects in your dog.

Benadryl Dosing for Dogs

The Merck Veterinary Manual advises pet owners that the proper dosage of Benadryl for dogs is typically between 2 to 4 milligrams (mg) per kilogram of body weight, administered two to three times daily. This equates to 1 to 2 mg per pound of body weight. The most common dose of Benadryl for dogs is 1 mg per pound. For example, a 25-pound dog would receive 25 mg per dose and a 75-pound dog would receive 75 mg per dose.

Tips for Giving Benadryl to Dogs

• Benadryl comes in tablets, capsules, and as a liquid.
• The liquid is generally easier to dose in small dogs.
• Capsules and tablets work great in large dogs, but if you have a dog between dose sizes, tablets are better because they can be split.
• When used for motion sickness, the dose should be given 30 to 60 minutes prior to travel.
• Ensure that the only active ingredient in the product you are giving is diphenhydramine.

The duration of administration depends on the condition being treated, your dog’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. Even if your pet feels better, the entire treatment plan should be completed to prevent relapse.

Dog owners are warned against administering time-release capsules to their pets. The American Kennel Club (AKC) notes that dogs 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.

NEVER give a diphenhydramine or a Benadryl®-based product that is combined with other ingredients without the approval of your veterinarian. Ensure that the only active ingredient in the product you are giving is diphenhydramine. Some combination medications contain additional ingredients that can be toxic to your dog.

Routine drug monitoring is not generally required in dogs receiving Benadryl.

Benadryl should not be given prior to allergy testing as it can interfere with some test results.

Benadryl is one of the human medications commonly recommended by veterinarians. Learn more about human medications commonly given to dogs here.

If you miss a dose of medication, give it when you remember. Do not double the next scheduled dose. Call your veterinarian for additional recommendations.

Overdose of Benadryl for Dogs

While generally safe, overdoses of diphenhydramine can cause various symptoms depending on the dose given relative to the weight of the dog. Signs of a severe overdose include tremors, seizures, lethargy or agitation, decreased appetite, weakness, vomiting, increased heart rates, low or high blood pressure, trouble walking, diarrhea, coma, and death. If you suspect your dog received an overdose of diphenhydramine or you observe these symptoms, please call your veterinarian, closest emergency clinic, or pet poisoning hotline.

Treatment will depend on the severity of the symptoms and may include induction of vomiting, gastric lavage, administration of activated charcoal, intravenous fluids, and muscle relaxants.

FAQs About Benadryl in Dogs

Does Benadryl need to be refrigerated?

No. Benadryl should be stored at room temperature out of direct sunlight and in locations with low humidity.

Will Benadryl make my dog sleepy?

Benadryl can make some dogs sleepy, but this is not a primary effect. For pet owners looking for sedation, there are other medications that are more effective.

How much Benadryl can I give my 60 lb. dog?

The most common dose given to a 60-pound dog is 1 mg per pound. Because it commonly comes in 25 and 50 mg sizes, a dose of 50 mg would be a good starting point.

Can I give my dog 50 mg of Benadryl?

You can generally give 50 mg of Benadryl to dogs that weigh 50 pounds or more.

How long does Benadryl last in a dog?

The effects of Benadryl generally last 8 to 12 hours and, therefore, the drug is given two to three times daily.

Is it ok to use Benadryl spray on a dog? Why or why not?

Various Benadryl sprays are on the market for humans with insect bites or problems like reactions to poison ivy. It can be used on dogs, however, some have an alcohol base which can cause drooling when licked. There are generally more effective topical products on the market to address skin problems in dogs.

Is it safe to give my dog Benadryl?

It can be safe to give dogs Benadryl, but only if recommended by your veterinarian. See precautions and dosing information above in this article.

Can I give my dog Children’s Benadryl for his allergies?

Many children’s Benadryl products are liquids and can be easier to dose for small dogs. For example, some are 12.5 mg per 5 ml of fluids. It is important to ensure the product ONLY contains Benadryl and no other active ingredients.

How much Benadryl can you give a dog for anxiety?

The dose of Benadryl for dogs is generally 1 milligram per pound of body weight. While you can give Benadryl for anxiety, there are more effective products on the market. Please discuss your dog’s anxiety problem with your veterinarian.

Can you give your dog Benadryl for a cough?

Benadryl is generally not an effective product for treatment of a cough. The best recommendation is to see your veterinarian to determine the underlying cause of the cough and receive the appropriate treatment.

How much Benadryl is safe for a dog?

The most common dose of Benadryl prescribed for dogs is 1 mg per pound of body weight.

Benadryl does work for my dog, but needs to be a little stronger…

It is common for Benadryl to not be enough to treat some conditions such as chronic allergies or allergic reactions. There are several antihistamines on the market and some dogs respond better to different ones. What doesn’t work in one dog can work better in a different dog. Other antihistamines recommended by vets are cetirizine (Zyrtec®) and Loratadine (Claritin®), both of which can be more effective in some dogs. See your veterinarian for recommendations based on your dog’s condition.

How much liquid Benadryl do I give my 8 lb. dog?

An 8-pound dog can receive 8 mg of Benadryl. If you are using a liquid product at the common concentration of 12.5 mg/5 milliliters (ml), then each ml is 2.5 mg, and the dose would be 1.56 (1.5 ml) to get 8 mg using this concentration.