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Featuring Demian Dressler, DVM and Susan Ettinger, DVM, Dip. ACVIM (Oncology), authors of The Dog Cancer Survival Guide

Benadryl For Dogs With Mast Cell Tumors: How It Can Help

Updated: November 18th, 2019


Vets often prescribe Benadryl for mast cell tumors. Why on earth do they do that? Dr. Dressler explains how this common allergy med can help.

Benadryl for mast cell tumorsBenadryl for mast cell tumors? What on earth does an over-the-counter allergy medicine have to do with cancer?? Let’s find out.

Most of us have heard of Benadryl, the brand name for diphenhydramine hydrochloride. It’s an antihistamine, which means it reduces the effects of excess histamines in the body.

In human medicine, we usually use it for allergies: hay fever, runny eyes, sneezing, and allergic sinus congestion. It is also used for more sudden-onset allergic reactions, including hives, facial swelling and so on.

In veterinary care,  we use it to treat things like insect stings, vaccine reactions or other allergic reactions in dogs.

And we often recommend Benadryl for dogs suffering from mast cell tumors.

Dogs with cancer are not experiencing allergies. So, what’s the logic?

Dr. Dressler goes into deep detail on Mast Cell Tumors in his audio seminar.

Well, it turns out there are similarities between a dog with a high mast cell burden and a dog experiencing allergic flare-ups. And Benadryl for mast cell tumors can address those.

Mast Cells Are Weird and Cool

Mast cells are really interesting cells. They are part of the immune system, a type of white blood cell.

Mast cells help rid the body of foreign invaders or material. They also help in healing. They do this in part by generating a substance called histamine. Yup, the same histamine that we talk about when it comes to allergies.

Mast cells that are normal release histamine as part of their work. Histamine helps attract other white cells to trouble spots, like foreign bacteria, or viruses, or injuries. They’re like those red flags your car’s roadside safety kit has. Histamine “marks” an area of the body that is in trouble, so other immune cells can find the area.

Histamine also causes blood vessels to dilate (get larger), and get a little “leaky.” The increased blood flow brings more white blood cells more quickly, and the little leaks help the white blood cells slip out of the blood and into the area that needs them.

So, you see, histamine is an important part of our immune system.

Mast Cell Detects Trouble ==> Releases Histamine ==> Attracting Other Immune Cells ==> Which Kill Invaders and/or Repair Injuries

So, yay histamine, right?

Yes. Mast cells are cool, because they produce histamine. Until they produce too much.

Mast Cells Can Overdo It

As with all things, balance is needed when it comes to histamine. Too much histamine is not good for the body.

When mast cells release too much histamine, whether during an allergic reaction or by huge numbers of cancerous mast cells, bad things happen.

  • The blood vessels in the area of the mast cells dilate, bringing inflammation and swelling of body tissues. In allergic reactions, this looks like hives and abnormal swelling.
  • The excess histamine is capable of causing the lining of the stomach to produce excessive acid. This causes loss of appetite and lethargy.
  • Blood pressure can be affected if enough histamine is secreted. It can drop to truly dangerous levels as many blood vessels open up all at once.

As it turns out, when a lot of mast cells are busy releasing their contents, they secrete lots of things, not only histamine. There are other chemical signals that work in tandem with the histamine, and those get released, too.

The end result of all these mast cells releasing their chemical signals is that the dog’s body goes into a completely abnormal state.

Inflammation and swelling results, and discomfort, as well. I would imagine the sensation is like getting stung or perhaps old stings (itchy, burning). Those with allergies might be familiar with the feeling of all-0ver discomfort, a sort of panicky itchy weird feeling. I imagine it is something like that for our dogs who have a massive release of histamine.

Histamine and Mast Cell Tumors

Now, both cancerous and non-cancerous mast cell tumors are capable of releasing histamine, and both are capable of releasing too much.

If your dog has allergies, you might see a little swelling or redness, and itching. Same is true if your dog has mast cell tumors.

But NOT every mast cell tumor will release very high levels of histamine that cause terrible symptoms like those above.

So if your dog has mast cell tumors, don’t assume that you will see those symptoms.

Only the most severe cases will have these high levels of histamine release.

Why We Use Benadryl for Mast Cell Tumors

Benadryl blocks the effects of histamine in the body, which is why it’s called an “antihistamine.”

Basically, it helps the body cope with abnormally high histamine levels.

That’s why you might hear your veterinarian recommend it for your dog with mast cell tumors: it can help to block the uncomfortable-to-dangerous effects of excess histamine released by cancerous mast cells.

Benadryl can help with the side effects of excess histamine sometimes associated with mast cell tumors.

Remember how excess histamine can also affect the stomach, by causing extra stomach acid? Benadryl helps with that, too. However, sometimes veterinarians will also suggest using an antacid as well, just to be sure to protect the tummy. Cimetidine and famotidine (Tagamet and Pepcid) are often used, and ulcers (if they are present) are often treated with misoprostol (Cytotec) and sucralfate (Carafate).

Diet Matters

If you’ve read my book, you know that I recommend a low-carb, relatively high fat, moderate protein diet for dogs with cancer.

If your dog with mast cell tumors is also experiencing excess histamine release — and remember, not all do — there are special dietary considerations you should also be aware of. You can read more about special nutritional choices for dogs with mast cell tumors in this article.

Of course, diet is just one of five steps in my Full Spectrum approach to cancer care. In addition to diet, nutraceuticals, anti-metastatic supplements, and mind-body strategies, you can find an entire chapter on the conventional care for mast cell tumors in chapter 30 of The Dog Cancer Survival Guide.

All my best,

Dr D

If your dog has mast cell tumors, this book is a wealth of information. In addition to the main steps Dr. D recommends, read the extra chapter dedicated to mast cell tumors from Dr. Ettinger, his oncologist co-author.


Discover the Full Spectrum Approach to Dog Cancer

Leave a Comment

  1. Gregg Whitman on December 28, 2019 at 5:05 am

    My dog has mass cell cancer her cancer doctor has her on benadryl 6 a day.She is swelling so bad she can hardly walk should we stop the benadryl.

  2. JLee on October 25, 2019 at 1:50 am

    How do they determine whether it’s grade I, II or III of MCT?

  3. Michelle Potter on August 31, 2019 at 10:48 am

    My dog had a mast cell tumor earlier this year. It was removed and he had chemo because it was a high grade two. Should he take antihistamines to avoid getting more?

    • Dog Cancer Vet Team on September 2, 2019 at 8:39 am

      Hello Michelle,

      Thanks for writing and we’re sorry to hear about your boy. Benadryl isn’t a preventative. As Dr. D writes in the article above, “Benadryl blocks the effects of histamine in the body, which is why it’s called an “antihistamine.” Basically, it helps the body cope with abnormally high histamine levels. That’s why you might hear your veterinarian recommend it for your dog with mast cell tumors: it can help to block the uncomfortable-to-dangerous effects of excess histamine released by cancerous mast cells.”

      But if you are wondering whether to use Benadryl alongside your boy’s current treatment plan, that’s a question best left answered by your vet. They know your dog, and your dog’s current treatment plan, and can make recommendations 🙂

  4. Dottie lohr on August 6, 2019 at 7:44 am

    How much Benadryl can a dog have in 24 hours. He is 75 lbs. and 12 years old.

  5. Sondra Revoy on June 9, 2019 at 6:29 pm

    I just located you scrolling thru google and my tears won’t stop! Bentley, my 10 yr old OES has had these “bumps” inside and on top of his skin throughout his life. It was suggested not to remove them until a procedure requiring anesthesia was necessary. Also the internal lumps have tested benign. You have given me a new direction, for knowledge is strength and everything I can apply to Bentley through your research is a gift from above. Thank you for being here so we could find you

  6. Nat. C on May 26, 2019 at 2:48 am

    Both our regular vet and consulting holistic vet suggested we don’t need to give our dog daily antihistamines, rather continue on his extremely healthy diet and top with a few other things often spoken about for every dog with a MCT.

    Pathology came back with subcut grade 0-1, MCI<1 with clear (albeit narrow) margins. Is this the reason they are not suggesting daily medication?

    We go back three monthly for check up of lymph nodes & earlier if we suspect any new lumps/bumps.

    Thank you.

    • Dog Cancer Vet Team on May 27, 2019 at 10:37 am

      Hey Nat,

      Thanks for writing. Unfortunately, we don’t know why both of your vets are against giving your dog daily antihistamines because we’re not vets, and we don’t know your dog or your pup’s treatment plan 🙂

      In this article, Susan writes, “In some mast cell cancers, large amounts of histamine are released in the body, causing irritation and itching. Most treatment protocols include an antihistamine to help, but many foods either have histamines or trigger the release of histamines in the body — so the diet must be looked at, as well.”

      In the article above, Dr. D writes that “Benadryl may be used to help with the side effects of excess histamine sometimes associated with mast cell tumors.”

      It’s possible that your dog may not be releasing enough excess histamine for your vets to justify adding an antihistamine into your dog’s current treatment plan. But as we’re not vets, we simply don’t know their reasoning.

      However, this is definitely the type of question that you should ask your vets 🙂 Find out why they don’t recommend an antihistamine.

      It’s okay to ask questions! You are your dog’s guardian and you should be made aware of why they do, or don’t, recommend particular treatments 🙂

  7. Leah on May 23, 2019 at 11:16 am

    Can we use Claritin too? Is it dangerous?

    • Nicole on June 13, 2019 at 11:17 am

      I have this question as well. My dog (who now has a MCT) had allergies when he was younger, and we were told that they recommend Claritin for larger dogs (mine is ~65lbs), since it’s stronger than Benadryl. We give him 1/day.

      • Molly Jacobson on June 14, 2019 at 1:56 pm

        Hi Nicole! I’m not a veterinarian, but I’ll jump in with my understanding, which is that the point of using Benadryl is to reduce histamine overload in cases that probably have it (mast cell tumors). Since Claritin is also an antihistamine, I presume that using it to reduce histamine would have the identical benefit. Ask your veterinarian about your dog’s specific case, but my thought is that if he’s already on Claritin as a routine way to control allergies, there would be no specific need to switch to Benadryl.

  8. Dawn on May 6, 2019 at 8:10 am

    My boxer was just diagnosed with grade 2 MCT for the second time. I can not give a high fat diet because she has pancreatic disease. Is there anything else you can recommend? Also, how much and for how long can you give the Benadryl for?

    • Dog Cancer Vet Team on May 6, 2019 at 9:06 am

      Hello Dawn,

      Thanks for writing! In general, the diet that Dr. D recommends in the Dog Cancer Survival Guide can be given to most dogs with cancer. However, it will need to be modified for your dog due to the high-fat content. Your vet will be able to make recommendations on what to include and exclude 🙂

      You may also find this article on Food and Nutrition for Dogs with Mast Cell Tumors to be beneficial 🙂

      Unfortunately, we can’t find any information in Dr. D’s writings on how long to use Benadryl, and what dosage to give your dog. This would be a good question to ask your vet as they will be able to tell you if Benadryl can work alongside your dog’s current treatment plan and at was dosage 🙂

  9. Laurie Mcdougall on March 19, 2019 at 5:40 am

    Hi. I bought Dr Dressler’s book when my “Mikey”, a 10 year old toy poodle was diagnosed with a mast cell tumor. It was surgically removed and luckily came back as level 1 with clean boarders. My vet said no further care was needed. I started making Dr Dressler’s recipe and added the Ever Pup and Apocaps.
    My question is, Mikey was previously on a Urinary So prescription dog food. I still use a little the hard kibble with the Dr’s recipe, is this ok for him since he had stones that he passed without needing surgery about five or six years ago? Thank you! Laurie

    • Molly Jacobson on March 20, 2019 at 4:01 pm

      Hi Laurie! It’s best to check with your veterinarian about diet related to your dog’s specific case. I’m not a vet, but my understanding is that kidney issues usually get a low-protein diet, so you might want to have your vet take a look at Dr. D’s diet and tweak it if it’s necessary. So happy to hear your Mikey had a good outcome, and I think you are wise to be vigilant and make changes anyway. Best of luck!

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