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

Spay/neuter and the association with cancer in dogs: part one

Updated: February 8th, 2021


Spay neuter dog cancer: despite its widespread popularity and the very good reasons to spay/neuter, it’s not without long-term risks to dogs.

In the US, there is widespread recommendation for early spay and neuter. But recently the association of spay/neutering and cancer in dogs has been in the news again. Specifically the concern is that spay/neutering increases the risk of cancer, which brings into question this recommendation to spay/neuter at 6 months of age. We are going to look into this complicated issue is this series.

I started to learn more about this when I joined Dr. Dressler and we began to prepare and edit the 2nd edition of The Dog Cancer Survival Guide. The studies were interesting, but contradicted what I learned in vet school at Cornell.

This was also just AFTER I had spayed my own seven-month-old Labrador puppy, Matilda. I began to question my decision as I explored the pros and cons of early spay/neutering. Let’s see why.

Spaying Female Dogs Lowers Breast Cancer Risk

We have known for awhile that hormones may promote or inhibit cancers, depending on the sex of the dog and the tissue or organ.

Less exposure to female sex hormones has been shown to be protective against breast cancers in dogs, which is important since mammary cancer is the second most common form of dog cancer.

Breast cancers can be nearly completely eliminated by spaying a female dog before the first heat. (On average, the first heat arrives at six months of age and recurs approximately every six months until late in life.)

Spaying provides less protection for canine breast cancer with every passing heat. Studies have shown that dogs spayed before the first heat have a 0.05% risk of developing mammary cancer, when compared to intact female dogs, which means the risk is almost completely eliminated by the surgery.

If the spay happens between the first and second heats, female dogs still have quite a bit of protection, with only an 8%  risk, compared to their intact sisters.

If the spay happens between the third and fourth heat, female dogs have a 26% risk, compared to intact dogs.

Clearly, spaying a female dog reduces her risk of mammary cancer. It also removes all risk for ovarian and uterine cancer.

So based on this, which is what I learned in vet school, I should feel good about my decision to spay my Matilda, right? We will come back to that.

For more helpful information and tools for dog’s with cancer, get a copy of the Dog Cancer Survival Guide

Neutering Males Lowers Testosterone-Related Tumors

What about male dogs?

Neutering definitely decreases the risk of benign perianal adenomas, which are stimulated by testosterone. As testosterone levels increase, glands called sebaceous glands get bigger, and so dogs can get these benign perianal adenomas. (Perianal means beside the anus.)

These benign butt tumors are seen most commonly in certain breeds, including Arctic breeds, Beagles, Cocker Spaniels, Shih Tzus, and English Bulldogs.

In addition to lowering the risk of developing benign adenomas, neutering your dog also eliminates his risk for testicular cancer. Testosterone-secreting tumors in the testicals can also lead to those benign butt tumors. Interestingly, perianal adenomas have also been reported in female spayed dogs who have with adrenal tumors (yep, you guessed it, they secrete testosterone).

The treatment of perianal tumors is castration. Remove the source of testosterone, and the tumors often resolve. If the tumor does not go away or significantly shrink, then we recommend removing the anal gland tumor. In fact, >90% of dogs are cured with castration and/or tumor resection.

Other Benefits of Spay/Neuter

In addition to the benefits above, spay/neuter also helps to reduce uterine infections (pyometras), and, in some cases, unwanted behaviors like humping, marking, and aggression.

Early spay/neuter is also important in population control and preventing the euthanasia of unwanted pets.  If you just look at these factors, early spaying and neutering might seem like an obvious choice.

But … Spay/Neuter Increases Risk for More Aggressive Cancers

But recently, there is evidence that less exposure to sex hormones, while protecting against the cancers named above, actually increases the dog’s risk for other aggressive cancers, including osteosarcoma, bladder transitional cell carcinoma, prostate cancer, lymphoma, and mast cell tumors.

And to further complicate the mammary tumor and early spay recommendation, this recommendation was recently questioned in an article called, Effect of neutering on the risk of mammary tumours in dogs – a systematic review (Beauvais, JSAP, 2012).

In a review of the studies that were used to make the recommendation for early spay/neuter, the evidence in favor of the practice was judged to be weak.

The article stated: “Due to the limited evidence available and the risk of bias in the published results, the evidence that neutering reduces the risk of mammary neoplasia, and the evidence that age at neutering has an effect, are judged to be weak and are not a sound basis for firm recommendations.”

So … maybe what I learned in vet school about early spay/neuter was not so straightforward after all! Join me for my next blog to learn how sex hormone can be PROTECTIVE against certain cancers.

Live longer, live well,

Dr Sue

Other Articles in This Series

Spay/neuter and the association with cancer in dogs: part two

Spay/neuter and the association with cancer in dogs: part three

Further Reading

Beauvais W, Cardwell JM, Brodbelt DC.  The effect of neutering on the risk of mammary tumours in dogs–a systematic review. J Small Anim Pract. 2012 Jun;53(6):314-22. doi: 10.1111/j.1748-5827.2011.01220.x. PMID: 22647210.

Leave a Comment

  1. Kathy Baker on February 23, 2019 at 4:31 am

    Thank you for your helpful and fair article. As a breeder (my dogs and pups live in our home, never in separate facilities), I cherish our dogs and constantly wonder if what I am doing can harm them. They are happy, healthy, and never seem to be troubled by having a litter. They recover quickly and produce delightful pups that have so far not developed any health issues. Even so I have been chastised for contributing to the problem of unwanted dogs by producing more pups. I actually do not agree with this as not every family is suited for a rescue. I donate to the humane society and rescues but do not feel that I’m the reason they exist. People have told me breeding our dams is shortening their life and putting them at high risk for cancer. My vet assures me that as long as they are well cared for this isn’t accurate but it’s so helpful to see an article that doesn’t just go with the “spay/neuter is always best” theory.

  2. Roger Vallieres on February 22, 2019 at 12:18 pm

    Spaying of Newfoundland at what age.

    • Molly Jacobson on February 23, 2019 at 8:22 am

      Hi Roger. This post will help you to understand what ages Dr. D recommends for spay/neuter in order to lower the risk of cancer. Of course, the decision about when or whether to spay/neuter any dog is best decided on a case by case basis, and certainly with the consultation of your veterinarian. The breed of the dog is less important than you might think, although there is evidence that purebred dogs of all breeds have a higher risk of cancer due to early spay/neuter.

  3. Lisa on July 6, 2018 at 6:12 am

    I’ve been in some debates regarding when to spay/neuter. My only position is wait until growth plates have closed and not that it shouldn’t be done. Recently someone brought up the mammary tumor issue and here’s a study that came out in 2015 — 10.1371/journal.pone.0127381. Again, I still believe one study does not or should not make new protocol but what are your thoughts on this study?

  4. Iren Abedi on September 17, 2015 at 6:52 pm


    Thank you for your caring article. I currently have a 4 month old toy poodle puppy and I just can’t seem to really understand why everybody tells me that she HAS to be neutered in order to not get cancer and I keep reading everywhere that a lot of dogs have lived a long life without being neutered or even gotten the extra vaccine shots. I believe that they were made with a natural immune system like every other animal and we do not get to decide, and it is not our position, to decide to remove a body part that does not even have enough evidence to prove that it causes cancer! According to that article 25% of dogs are able to NOT get cancer from being spayed or neutered. And I really believe if a more systematic and precise study was made on this subject the percentage would be much much lower. Think about it, pharmaceutical companies are really the ones controlling our market and are making so much money off of us because we’re sick from all their GMO and cheap junk food. You can see why BAD food is cheap and good quality organic, natural food is so expensive. This is a very short story about what we humans as the most “intelligent” animals are going through. Imagine how much money those companies can make BESIDE us? Most of us would do anything for our animal friends and would spend thousands of dollars on them just to have them with us longer. With that in mind, why not do research and conclude what makes sense to you? We do not always have to follow the crowd. We need to question things and improve our health and way of living. I really don’t understand how people get offended by the truth sometimes. I just wanted to thank you for being human enough to bring up this topic.

  5. Linda M on February 7, 2015 at 9:00 am

    The increase in Mammary Cancer in this country has a great deal to do with feeding GMO corn and GMO corn gluten in dog food. A study was done in France feeding rats GMO corn for a two year period. Every single rat developed Golf ball size Cancerous Tumors in mammary glands predominantly. Monsanto knows this and won allow testing in the US over a few months. I have raised dogs for well over 40 years never spay or neuter and never suffered cancer in one dog. Feeding exceptional food has a great deal to do with the continued health of your dog. Timberwolf Organics is one such food that is well made and produces excellent health. The idea of spaying to remove the threat of cancer is like telling women they should have mastectomies, and hysterectomies to make sure they wont succume to cancer> How about being aware of supplying healthy foods and proper nutrition to build the immune system and a sound resistance in our animals rather then pre removing body parts? The stats on destroying the natural hormone system in any animal is already established and a poor option for a multitude of reasons

  6. I Hate Mustard on July 22, 2014 at 8:05 am

    I’ve bred Australian Shepherds for 20+ years and only one of the pups I produced had her first heat cycle before one year of age. Mine typically have their first heat cycle around 14 – 18 months. With all of the research I’ve done over the years my contract stipulates that my dogs get altered at 1 year of age, but NOT at the time of the rabies vaccine. I got an email from one of my puppy buyers a couple of days ago. Their dog died at 17 1/2!

  7. sardoglady on May 29, 2014 at 5:17 am

    Usually, the initial heat occurs at 9 months not 6 as stated in the article. I’ve had GSDs wait until 14 months for the initial heat.

    • Susan Kazara Harper on May 29, 2014 at 5:35 am

      Thanks for this. As with everything, statistics are important, yet not set in stone. My girl started her first heat at 5 months ( she was, of course, very advanced for her age 🙂 We need to be informed, and let our dogs be the individuals they were born to be, caring for them all the way.

  8. Heidi Flowers on April 15, 2014 at 4:14 am

    Perfectly said Pam!! Shame shame on Dog breeders for purposefully bringing more Dogs into the world… and WHY? …For money and prestige.

    Shame shame on the people who SHOP for a made for them Dog, rather than ADOPT one who is homeless and crying out for LOVE.

    In the US alone MILLIONS of Dogs are euthanized every year. I have seen too many photos of garbage cans full of dead unwanted Dogs. I am with Bob Barker on this one… BUT I certainly AM for less invasive spay/neutering… and NOT too young, that’s for sure.

    Absolutely perfectly said, Pam, on the fertilizers too! Eat your dandelions instead of spraying AGENT ORANGE (YES!!!) . In addition, feeding Dogs foods from FACTORY raised Animals, along with contributing to promoting the horrid torture of these suffering creatures… ALSO passes a plethera of HORMONES, Anti-biotics, and other CHEMICALS (glysophate/round-up) to your Dog… THOSE all cause CANCER.

    Having said all this, I adopted a Lab/Rottie, at age 4, and suspect due to a tiny incision mark, that she had been spayed quite early. She was fed healthy free-range kibbles, we don’t use chemicals around here… but she developed bone cancer at age 10. I believe it was due to her too early spay. She also had issues with her CCL’s.

  9. Garry Caswell on October 6, 2013 at 8:56 am

    Dear Dr. Ettinger, First of all thank you for all you do, I write you with crying eyes, we had our Rottweiler Cait spayed around 6 months of age for all the same reasons you did Matilda, however being a new dog owner (first dog as a adult) we were unaware of the side affects of spaying at an early age, and I’m starting to get more and more upset with my vet for not telling us about the damages of spaying. Now, Cait (5 years old in January 2014) has a lump on her leg (at the hock) took her in and our vet x-rated the area and it showed a mass built up around the bone, which “might be” cancer but needs to drill into the bone to take a sample, BTW, Cait did hit this same area on a concrete table about a week earlier so we hoping that it was just a bone bruise, so the Vet put Cait on antibiotics for two weeks and we are due back to the Vet tomorrow. Is it common for a Rottweiler of Cait’s age to get cancer?

    • DrSueCancerVet on October 13, 2013 at 10:22 am

      Remember that intact dogs can get cancer too, so try no to blame yourself. This is relatively new info about the potential protective benefits of sex hormones, and we still have a lot to learn about it. With that said, primary bone cancer is most common in middle aged and older dogs but there is a second peak incidence in dogs that are 18 to 24 months of age. With the history of trauma in that area, I do hope it is something benign, but you should have it worked up. Also discuss starting maybe with a bone aspirate. Have you found my blogs on osteosarcoma? Also check out the book, chapter there too. Good luck to you and Cait.
      All my best, Dr Sue

  10. Pam Wilkinson on June 6, 2013 at 4:43 pm

    I really feel you should start your spay/neuter article with: In the U.S. over 14,000 dogs and cats are killed/euthanized every day simply because they were born unwanted.
    Spay and neuter is the most important so all dogs and cats can live a long healthy life. Piglet at 18 years old does have cancer. She was not spayed early actually when I found her on the streets she had a litter. The pups if not spayed/neutered can procreate over 30,000 dogs in several years. Over half will land up at shelters likely to be euthanized or if they land up on the streets they will die miserably.
    I really wonder where you got the info that early sterilization causes cancer. The grass fertilizer causes cancer. Your better to tell people don’t use fertilizer and feed your dog greens and healthy food. This prevents cancer as I have learned from you. I want to be removed from future emails as you give the red carpet for people not to sterilize their pet until it’s possibly too late and another unwanted litter is born to be killed.

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