When should you neuter/castrate your male dog? Should you wait till after a heat cycle to spay your female dog? These used to be questions with easy answers, back when most veterinarians advised the surgery after 6 months of age. Science has muddled that simple directive.

A 2020 study in Frontiers of Veterinary Science found the age at which we neuter our dogs—with neuter meaning “surgically sterilize” the dog so he or she cannot reproduce—may affect the dog’s lifetime health. This study cited large-dog joint health, the risk of developing cancer, and female urinary incontinence as possible problems from spaying or neutering too early in the dog’s life. That said, the researchers found it ultimately depends upon the breed of the dog: “Guidelines that avoid increasing the risks of a dog acquiring these joint disorders or cancers are laid out for neutering ages on a breed-by-breed and sex basis,” according to the researchers. In other words, there is no “one age fits all” when it comes to neutering or spaying.

An earlier study published in The Journal of the American Veterinary Association also found that the age of neutering or spaying may affect the dog’s joint health—again for large dogs—saying, “In general, large- and giant-breed dogs were more likely than other dogs to have HD (hip dysplasia), CCLD (cranial cruciate ligament deficiency), or both” due to early neutering. Some veterinarians advise waiting until 24 months of age for giant-breed dogs.

And, of course, cancer risk in Golden Retrievers is a well-known concern, with a lifetime study funded by the Morris Animal Foundation. The chance of cancer in female Goldens is so high it almost makes the issue more of “should we spay” rather than deciding what age to spay.

It’s Hormones, Baby

What’s this all about, really? Hormones. Whether your dog is male or female, the hormones testosterone and estrogen matter to your dog’s physical and, possibly, mental development. These hormones are made in the testicles or ovaries of your dog, so if you remove these organs, the hormone production stops.

For example, estrogen helps with muscle tone in the urethra, which is why some female dogs develop urinary incontinence. And testosterone can cause abnormal prostate overgrowth in an intact male dog and impact rectal function, possibly blocking the path of feces through the rectum and requiring emergency surgery. One good reason for a late spay, but a looming concern for a late neuter. And, of course, if you have a canine athlete, these hormones help develop muscle growth and, possibly, even drive in your dog.

When to Spay Your Female Dog

The decision on when to spay your female dog is not easy. Discuss with your veterinarian what you want to do with the dog and your ability to handle a heat cycle (male dogs will seek her out and she will want to “meet” them!). While vets don’t have crystal balls, they are familiar with the research on these issues and have a much wider base of experience than you probably do.

In general, if you have a small female dog (under 40 lbs. projected adult weight), you have more leeway in choosing when to spay because smaller dogs seem less affected by all these health concerns than larger dogs. If you want to avoid her having a heat cycle, many veterinarians will spay her at 6 months of age. Research, as of yet, shows no problem with this decision.

If you have a larger female dog, you need to weigh the developmental risks of an early spay—as shown by research—against dealing with a heat cycle. At about 1 year of age, most veterinarians will advise you that it’s acceptable to spay as your dog is fully grown at that age, making the lack of hormone production less likely to be a problem. That could be only one or two heat cycles to endure.

It’s important to know, however, that the risk of mammary tumors increases in unspayed female dogs. That’s because intact female dogs have fully developed mammary tissue. A dog spayed prior to sexual maturity (around 6 months old) is at a reduced risk of mammary cancer because the spay removes the ovaries, which produce the estrogen that is significant to the full development of the mammary glands, according to Cornell DogWatch.

The Veterinary Society of Surgical Oncology (VSSO) says mammary tumors in female dogs account for 42% of all diagnosed tumors, with a lifetime risk of 23% to 34%. “Compare that to women, who have a 12.4% lifetime risk according to Susan G. Komen, an American breast cancer organization,” says Cornell DogWatch.

“At a cellular level, mammary tumors in dogs and cats are very similar to human breast cancer tumors,” says Dr. Scott Coonrod, the Judy Wilpon Professor of Cancer Biology and director of the Baker Institute for Animal Health. “And they may occur in pets even more often than they do in humans.”

When to Neuter Your Male Dog

The decision on when to neuter your dog again may depend upon what you’re going to do with the dog and your ability to handle an intact dog. As with female dogs, smaller breeds are less affected by the study results, which show larger dogs have more problems. With a big dog, your veterinarian may advise you to wait till 12 to 24 months of age to neuter your dog and ensure skeletal growth is finished. However, if you are having any temperament/aggression issues with your male dog, early neutering may be wise. Testosterone is linked to aggression.

How Much Does It Cost to Spay or Neuter Your Dog?

Not surprisingly, the cost of a spay or neuter is a regional thing. Prices in metropolitan areas can surpass $500 for a spay and $250 for neutering (castrating). If you’re working through a rescue or animal shelter, not only will they require that the dog is spayed or neutered, but they will often have low-cost plans to help control the births of unwanted puppies.