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Featuring Demian Dressler, DVM and Sue 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

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  1. Leanne on February 23, 2020 at 7:35 am

    I’m just reaching for what to do to help my dog Remington from dying from cancer. He is 7 1/2 years old and is a French Bull dog- Beagle. He had 2 small grade II mast cell tumors on his lower right side of his torso near ribs and 1 larger subcutaneous mast cell tumor on his left knee removed 1/28/20. The Dr said per pathology report she did not get clean margins on the knee tumor and the “dirty margins” is located at the 6 o’clock area or bottom of the incision on his knee. I was giving him Benadryl twice a day am & pm all up until the surgery ( about 6 weeks ) and continued it after the surgery and the Dr added Meloxicam and Famatodine. He hasn’t been on the Meloxicam or Famatodine for weeks now. I stopped the Benadryl for a while because I felt bad making him sleepy all the time. I just started giving him Benadryl in pm again because of the literature on it helping cancer in dogs. His Dr said I could stop giving it to him if I want to. We haven’t seen Oncology yet. I’m not sure if I should be giving the Benadryl twice a day or not. If it will help or hurt. Because it is confusing, it sounds like histamines the MCT’s give off are good and not good. Prior to surgery he had x-rays of his lungs and thorax and ultrasound and aspirations of his spleen and liver and nothing was seen. But he has had a small unidentified nodule on his lungs for many many years and his recent recheck of the lungs showed 2. He started turning so gray very young and since we first noticed the tumor on his leg in November and the surgery he has turned so much more gray he looks very old. I want to do the right thing for him in caring for him right now. I’m afraid of the Oncology appointment after reading alot about subcutaneous MCT’s without clean margins on the limbs. I just want to do what I myself can do for him right now each day. I switched him to NW Naturals raw for his dinner 1/2 cup with 1/2 cup of Canidae Pure grain free Bison. In the morning I always cook eggs 1/2 cup and 1/2 cup of the Canidae. He had a lot of allergies/ ear infections and gooky eyes when younger so going with grain free has helped. I thought he might have an allergy to poultry but I’m not sure, and just avoiding it in fear it will make his histamines worse. Your advice on if it’s a good idea to give him Benadryl and Famatodine twice a day would be so appreciated. I also just read that exercise and being out in the cold causes inflammation in the area where the MCT’s are so I’m wondering if I should give him Benadryl before his walks out in the cold. I just want to understand it all better and make the right choices and I feel so lost. He had wonderful Dr. but she just says if you want to you can and doesn’t seem to have an opinion on it. I’m always afraid there’s a chance he’s dying with the cancer in him still and he has changed over the last 6 months. He gets up very late morning, eats and then goes back to sleep for several hours before wanting to go outside to walk ( or go to the bathroom ) He just holds it and sleeps..Sometimes, actually quite often he will not get up after his breakfast until 2 or 3pm. Once out he comes alive and seems to like to walk. 🙂 But he’s just not his old self and it of course worries me. Any help you can give I will appreciate so much.

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