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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. Jen Green on February 18, 2019 at 3:35 pm

    Hi, please can you advise re the recommended dose of Benadryl? My otherwise healthy 8 year old black lab has had 3 high grade mast cell tumours removed in the last 10 months. She weighs 21 kilos. Thank you.

    • Dog Cancer Vet Team on February 19, 2019 at 8:37 am

      Hi Jen,

      Thanks for writing! As we’re not veterinarians here in customer support, we can’t offer you medical advice. However, we can provide you with information based off Dr. Dressler’s writings! 🙂

      Dr. D answers this question in a previous comment. He writes, “The dose for benadryl is 1 mg per lb, 2-3 times daily, but check with your vet.”

      Please consult with your vet on what the correct dosage would be for your dog, as each dog, their health situation, and their treatment plan is unique 🙂

  2. Gavin Hatton on November 26, 2018 at 11:04 pm

    My name is Gav and I live in the UK my dog has had a mast cell tumour removal 2 years ago and now has 3 bumps appear on his side they have been the same size for a few months but last night became large in about 10 minutes then shrank in about 20 mins would a antihistamine help and how much , I am terrified to take him back to the vets but I know I’ll have to.his name is Ralf and he’s a staffie cross

    • Dog Cancer Vet Team on November 27, 2018 at 8:20 am

      Hello Gavin,

      Thanks for writing, and we understand how terrifying this situation must be for you. We’re not veterinarians here in customer support so we can’t offer you medical advice. However, we can provide you with information based off Dr. Dressler’s, and Dr. Sue’s writings

      As Dr. Sue writes in the article below, if a lump is 1cm or larger, or has been there for over a month, get it checked by a vet ASAP. This might mean getting a fine needle aspirate (or a biopsy in some cases) to determine what the lump is– it’s better to know sooner rather than later. Here’s the link to the article where Dr. Sue goes into more detail on this:

      Consult with your veterinarian 🙂 They know you, and Ralf the best, and will be able to check, and test what the lumps are, and they will be able to provide you with medical advice on what treatment plans/options are available 🙂

      We hope this helps!

  3. […] prescribed for such a thing, but then I read this article explaining mast cells and histamines. Why Benadryl For Mast Cell Tumors? – Dog Cancer Blog There are some interesting comments on the article from people going through this with their dogs. […]

  4. […] can release histamine and cause hives. What you should know about mast cell tumors in dogs: Part l…t-cell-tumors/ Last edited by Roccosmom; Today at 01:44 […]

  5. Susan Kazara Harper on June 23, 2015 at 8:22 pm

    Beverly, Bless your heart, I know this is hard. Why is she on Benadryl?

    • Beverly Johnson Carroll on June 24, 2015 at 5:36 pm

      She isn’t on Benadryl, I was readying the cancer Vet gives it to take the pain away for cancer, she was on Prednisone; but it made the cancer spread rapid. This is Wednesday night and Thursday morning we will be putting our precious Angel to rest her cancer has progressed so much and she isn’t resp to much. Thank you.

    • Beverly Johnson Carroll on June 24, 2015 at 5:39 pm

      She isn’t on Benadryl, I was taking the advice of the cancer Vet where he said he gave it for the histamine in the cancer. We will be putting our precious Angel to rest on 6/25/15. Her cancer has progressed.

      • Sheila Harris on July 10, 2019 at 4:21 pm

        My 10 yr old dog has had MCT for 2 yrs & we’ve done several treatments including surgery but they can’t get clean margins because it’s on her paw. We do Benadryl & prednisone almost daily but still fighting to keep it down. Is this safe? She’s gained so much weight & seems uncomfortable
        Not sure what to do next??

  6. Beverly Johnson Carroll on June 22, 2015 at 1:02 pm

    My Vet has my dog on prednisone, tramadal 50mg, and antibiotics for now until he knows for sure if she has cancer. She has a large mass on her left breast and a deviated heart. Can I still give her Benadryl? I need to help my baby.

  7. Susan Kazara Harper on November 25, 2014 at 12:33 pm

    Patti, We highly recommend the Dog Cancer Diet, which is in Dr Dressler’s book, the Dog Cancer Survival Guide, as well as at as a free download. There are some really good kibbles on the market now, and using them alongside a real-food diet will go a long way toward helping your girl thrive. Personally, I have learned to never assume the cancer is permanently gone… it only take a few cells and my not paying attention for awhile. Better, I feel, to remain vigilant, get the absolute best nutrition and support into my dogs and fill their days will joy and play. Don’t spend days worrying, but stay watchful and make a note of any changes. Take photos on your phone periodically if you think you see any changes. You may want to consider Apocaps, which was designed for dogs with conditions like cancer. will give you more information about how the ingredients support your dog. We are all sharing the happiness of your good biopsy results… congratulations. Give your girl a hug from our team, and don’t hesitate to get in touch if we can help in any way.

  8. diamondinruff on November 12, 2014 at 7:00 am

    My 3 year old chihuahua mix just had a large mast cell tumor removed from her perinanal area. The biopsy came back as Grade 1 and the vet said there didnt appear to be any spread anywhere. She is on benydryl – he said for the rest of her life. Now, I want to be sure she is eating the best diet for her life. What should I be feeding her? She was eating a good grain-free kibble.

    • Joanna on December 20, 2014 at 9:15 pm

      My izzie had a growth near her private area that took off growing from like a small marble size to the size of a fat cigar about 2 in long. I did not go to the vet I gave her RASPEX I had read about it a few years ago and lots of good reports of how it swank cancer / tumors in animals & left them clear So I thought to try it before going to the vet and I was so pleased. I prayed in Jesus name ,laying hands on izzie & started giving her 2 tabs in am & 2 tabs in pm. In less than 4 weeks it was almost gone. Not selling anything just sharing what I found & how it helped us. Hope this will help you. Lo

  9. Susan Kazara Harper on September 9, 2014 at 3:18 am

    HI Kathy,
    You can ask your vet about trying hydroxizine and mirtazapine- they’re not as effective as benadryl but you’re looking for the best balance for her. Also low dose prednisolone, but all with your DVM in the loop, and your vet is the best one to work with on dosage. I hope this helps!

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