Benadryl for Dogs: How, When, and Why it Works

Can you give a dog Benadryl? Although many over-the-counter human medications are not recommended, Benadryl for dogs has its place in veterinary medicine. Integrative veterinarian Dr. Julie Buzby explains how, why, and when Benadryl is used for dogs.

Can you use Benadryl for dogs?

Benadryl (also known as diphenhydramine) wins the award for being the human medication that my veterinary clients most commonly ask me about administering to their dogs.

I find myself frequently answering my clients’ questions about Benadryl use for allergies, anxiety, travel, and other doggie dilemmas.

The good news is that Benadryl (xylitol-free) is generally safe for most dogs. Unlike many other human medications that have potential for causing serious harm, oral Benadryl has a wide margin of safety.

Before you grab for the Benadryl, always check the label to make sure it doesn’t contain xylitol, an artificial sweetener that is dangerous for dogs. Also, Benadryl may be contraindicated in dogs with certain health conditions. We’ll get to more on both of these caveats at the end of this article.

Despite the fact that over-the-counter Benadryl is not FDA approved for veterinary use, it’s safe and helpful for treating several conditions in dogs including the condition the dog is presenting in photo below.

What do you think is going on with the dog in this picture?

If you guessed anaphylactic allergic reaction, you are correct! These types of reactions can occur due to anything from vaccinations to insect bites or stings. (If you know anyone who carries an Epipen, it’s because they too are prone to this life-threatening type of allergic reaction.)

Benadryl for dogs who’ve had an anaphylactic reaction 

Since I like to use my veterinary experiences to explain conditions and medications, let’s meet Cooper, a Pug who almost died in my arms after a routine vaccination. Immediately prior, Cooper’s family had checked out at our reception desk. Cooper was ambling at the end of his leash, sniffing the ground. I had waved goodbye and moved on to my next appointment.

As they prepared to leave, Cooper vomited. His dad felt embarrassed, apologized for the mess on the floor, and offered to clean it up. Thankfully, my astute receptionist quickly put the pieces together. She raced around the corner, took one look at Cooper’s swelling muzzle, scooped him up, and rushed him into our treatment area.

The Pug’s little gums were turning a sickening greyish color and his rectal temperature was starting to climb. To treat Cooper, he received an injection of diphenhydramine and a cocktail of other medications. Thankfully, the Benadryl and other medications did the job. Cooper went home that evening with no issues at all.

The most common and impactful use for Benadryl in veterinary medicine is the scenario I just described above—treating allergic reactions.

How Benadryl works in an anaphylactic reaction

Next, let’s look at the following five important points to understand how Benadryl works in an anaphylactic reaction.

1. Benadryl is an antihistamine. (We’ll use this information in point 5 below.)
   
2. Mast cells, which reside throughout the dog’s body, are a part of the immune system. They’re a type of white blood cell. Each mast cell is of full of tiny granules called histamine. We see mast cells in highest concentrations in a dog’s skin, respiratory, and gastrointestinal tract. The image below illustrates a mast cell.
   

3. When mast cells note a potential foreign invader, the cells trigger the immune system by “degranulating.” Degranulation dumps histamine and other powerful chemicals into the surrounding tissue to summon the rest of the immune system “army.”

4. Allergies are simply the body’s immune system overreacting and inappropriately ramping up to fight something that should not normally be considered a threat. Histamine and other chemical mediators dumped by mast cells cause the characteristic symptoms of allergies.

5. Benadryl, our ANTI-histamine, blocks receptors on the blood vessels and smooth muscle. This prevents histamine from binding to cells in the immune system involved in allergic reactions—avoiding the overreaction and allergy symptoms.  

Benadryl pre-medication for dogs before vaccinations

Now, back to Cooper. Three years later, when Cooper’s vaccines were next due, we prepared for the worst and again administered Benadryl. But this time, I had his parents administer oral diphenhydramine an hour prior to his appointment. I wasn’t sure if it would be enough to completely prevent a reaction, given the gravity of his previous reaction. I told his parents to plan to hang around the vet clinic for 45 minutes after the appointment so I could keep a close eye on him.

With Benadryl on board, Cooper was fine. He showed no signs of a reaction. He received a big gold star in his chart where I’d documented in detail both his allergic reaction and our success in “pre-medicating” him with oral diphenhydramine.

If your dog has ever had an allergic reaction to a vaccine, the first step is to determine what your dog reacted to, if possible. Avoiding the reaction entirely is better than treating the aftermath!

Sometimes, because some vaccines are “combo” injections, and because we often give multiple vaccinations in the same visit, we don’t know what exactly the dog reacted to. For dogs with a history of allergic reactions to vaccines, I will often split up their vaccines for future appointments, so we can be sure we know what they are reacting to if they have another reaction.

For any dog with a history of vaccine reactions (no matter how small), “pre-medication” with Benadryl is a critical part of the plan if and when vaccines must be administered again. If you are concerned about a vaccine reaction, speak with your veterinarian about whether pre-medication may help your dog.

Some veterinarians may recommend an appropriate dose of Benadryl at home before your appointment. Others may give your pet an injection of Benadryl at the appointment prior to the vaccines. Your veterinarian also may want to discuss whether certain vaccines should be given to your dog at all.

As a side note, because we now typically give pets vaccines less frequently than we did years ago, and through the use of titers in dogs, dogs who have vaccine reactions have more options than they once did.

In general, for a healthy dog with no previous history of vaccine reaction, the small risk of him or her having a reaction does not outweigh the risk of contracting a preventable disease. Routine vaccination is very important for preventive health! (For more on vaccines, please check out my article that outlines which dog vaccines are necessary.)

Benadryl for dog’s bee sting

Dog bee sting reactions can range from mild localized swelling to anaphylaxis. If your dog has been stung by a bee, giving him or her a dose of Benadryl is a safe and effective way to reduce the immune response and avoid a severe reaction. If you can, try to remove the stinger from your dog’s skin, and give a dose of xylitol-free oral Benadryl (with guidance from your veterinarian) as soon as possible.

If your dog suffers a bee sting and experiences a reaction that doesn’t get better with Benadryl, seek emergency veterinary care.

Benadryl for itching

Because it affects the immune system, Benadryl can help with some types of itching in dogs. Contact reactions to irritants such as poison ivy can improve with Benadryl.

The most common cause of itching in dogs is allergies. Dogs can experience seasonal allergies, food allergies, environmental allergies, and others. Benadryl can provide minor relief for some types of allergies, such as environmental allergies, but is ineffective against other types, like food allergies.

Giving Benadryl for dogs with environmental allergies (atopy)

Next, let’s look at the effectiveness of Benadryl for dogs with environmental allergies.

Environmental allergies are a common health problem for dogs. Remember, mast cells that trigger allergy symptoms are highly concentrated in their skin.

Unfortunately, while giving dogs Benadryl for seasonal allergies is a reasonable starting point, it’s not nearly as effective as other categories of drugs. Benadryl just doesn’t seem as effective for dog allergies as it is in people. In other words, while Benadryl played a role in saving Cooper’s life after his acute allergic reaction, Benadryl is unlikely to play the role of hero for dogs with environmental allergies.

Fortunately, there are many other allergy medicines for dogs beyond Benadryl for treating environmental allergies. Hyposensitization injections (“allergy shots”) and drugs like Apoquel® and Cytopoint® are just a few choices available. Since the days when I was in vet school, veterinary medicine has made great strides and we now have many safe, effective solutions available for dogs who suffer from atopy.

When it comes to environmental allergies in dogs, there is no one size fits all. This can be a challenging condition to treat, and sometimes treatment can involve a bit of trial and error to see what works for an individual dog. If your dog deals with environmental allergies, your veterinarian will work with you to come up with a plan to suit your dog’s specific needs.

Using Benadryl for dogs with mast cell tumors (MCT)

Now that we have a measure of understanding on Benadryl’s effectiveness for anaphylactic reaction and environmental allergies, let’s look at a third condition. Let’s understand Benadryl’s role in reducing side effects of histamine release from mast cell tumors (MCTs).

Recently, I met Zeus, a 10-year-old Boxer who presented to me for acupuncture for dogs. Before doing his acupuncture, I spent a few minutes talking to his parents about Benadryl. Why?

Zeus had recently been diagnosed with a mast cell tumor—the most common malignant skin tumor in dogs (though they can occur internally also).

Like the name implies, mast cell tumors are loaded with histamine-containing mast cells. This is why an antihistamine (Benadryl) can help reduce side effects of histamine release from MCTs.

If you’d like to learn more about MCTs, you can find a wealth of resources and information on Dr. Sue Ettinger’s website. Dr. Sue Ettinger is a practicing veterinary cancer specialist, international speaker, book author, and vlogger (video blogger). Also known as Dr. Sue Cancer Vet®, she is my go-to resource for the latest in cancer treatments for pets. She shared the image below as a quick snapshot on mast cell tumors. .

Is Benadryl an effective sleep aid for dogs?

As I alluded to above, one side effect of Benadryl is sleepiness. Sometimes pet parents will give their dogs Benadryl to creatively leverage this side effect—for car travel, during Fourth of July fireworks, or other occasions where the dog needs a mild sedative “to take the edge off.”

While sleepiness is a potential side effect, Benadryl is not actually an effective sleep aid by any means. In some instances that side effect can be used to a dog’s benefit. 

Essentially, giving Benadryl is the equivalent of drinking a glass of milk before boarding a plane to calm your nerves if you have severe anxiety about flying. Think very mild (if any) efficacy as a sedative. The sedative impact may decrease with chronic Benadryl use because a dog may develop a tolerance to this effect.

It is important to remember that Benadryl is not an anti-anxiety medication. If your dog experiences anxiety related to travel, noises (fireworks and dogs, for example), or other situations, he or she may benefit more from a medication that will address the underlying anxiety rather than just causing drowsiness. Discuss your concerns with your veterinarian, and they can offer appropriate solutions for your pup.

Is Benadryl effective to help a dog’s nausea when traveling?

On paper, at least, Benadryl is also an anti-emetic, which means a drug that helps prevent vomiting. I mentioned its use for dogs who are traveling. It can be helpful for dogs on road trips both by “chilling the dog out” a bit, but also because it may help with car sickness.

However, if your dog struggles with motion sickness, there are much better drugs available to address these concerns. Speak with your veterinarian about other options.

What dose of Benadryl should my dog take?

Benadryl dosage for dogs depends on what condition you’re treating, how many times a day you’re giving it, and your dog’s body weight. There are also various formulations of the drug—Benadryl tablets, liquid Benadryl, even time release formulations. This affects proper dosing too.

That’s why you should not give your dog Benadryl or any other over-the-counter medication without first asking your veterinarian if it is safe, and what dosage is appropriate for your dog.

A word of caution about Benadryl and dogs with health conditions

Benadryl may be contraindicated with certain health conditions. It should be used with caution with ailments such as seizures in dogs, heart disease in dogs, or high blood pressure (i.e. hypertension in dogs). Your vet will review what other medications your dog is taking, whether Benadryl could interfere with these medications, and help you determine the proper dosing for your dog.

Checking the Benadryl label is crucial for your dog’s health and safety

Please note, when administering Benadryl to your dog, it’s generally fine to use the human formulation that you buy over-the-counter. However, it’s vitally important that the Benadryl formulation not contain xylitol (an artificial sweetener commonly used in products intended for people) or any other ingredients that are harmful to dogs.

Also known as birch sugar, xylitol is toxic to dogs and leads to severe low blood sugar and/or liver damage.

Diphenhydramine must be the only active ingredient in the product. If you’re not certain whether the medication you have is safe for your dog, ask your vet.

Consult your veterinarian about giving your dog Benadryl

Finally, the good news about giving dogs Benadryl is that, when used properly, it is a safe medication. However, before administering Benadryl to your dog, I highly encourage you to check with your veterinarian.

What questions do you have about Benadryl for dogs?

Please comment below.