Spaying takes out the ovaries, usually with the womb. Neutering takes out the testicles. Both are done under a general anaesthetic, and most dogs and cats go home the same day. The question owners actually have is when, and for the first time the profession's answer is different for cats and for dogs, and different again for a Chihuahua and a Golden Retriever. Everything here comes from the guidelines vets work from and the studies that changed them, and each one is linked at the bottom.
A veterinary task force on feline sterilization met in 2016 and recommended that cats not meant for breeding be spayed or neutered by five months of age. The AVMA endorsed it in 2017, and the American Association of Feline Practitioners, AAHA, the Association of Shelter Veterinarians, the two big cat registries and seventeen state veterinary associations have since. It is the one spay and neuter question with a single answer.
The reasons are practical. A female kitten can be pregnant by five months, some by four, so the old advice of six months produced litters. An intact male starts spraying at about the same age, and the sexual behaviors are a common reason cats are given up to shelters. And the cancer numbers are the strongest in the whole subject: 80 to 90 percent of mammary tumours in cats are malignant, a cat spayed before six months has a 91 percent lower risk of one, before twelve months 86 percent lower, and spaying after two years of age gives no protection at all.
The joint and cancer findings further down are dog findings. The task force looked for the same problems in cats and found no evidence of any.
Kittens can be done from eight weeks, a female can be spayed while she is in heat, and an adult cat that was never done can still be done; past two years it no longer lowers the mammary risk, but the litters and the spraying still stop.
The traditional age for a dog was six to nine months, whatever the dog. Since 2013 a series of studies from the University of California, Davis has found that in some breeds, neutering in the first year went with more joint disorders, and in a few breeds with more of certain cancers, while in small breeds it made no difference at all. The guidelines vets work from now split by the dog's adult size.
Males at six months. Females before the first heat, at five to six months. That is the 2019 AAHA guideline, and the UC Davis data argued against it in only two small breeds, the Boston Terrier and the Shih Tzu, below.
When growth is complete, which is usually somewhere between 9 and 15 months.
Between 5 and 15 months, decided with the vet on the breed's own risks and her life. The guideline calls this the case that has to be individualised, because waiting past a heat trades one set of risks for another.
The AVMA's page for owners says the same thing in fewer words: for dogs there is no one-size-fits-all recommendation, and the breed, the sex, the job the dog does and the home all go into it.
The study most vets are quoting is from 2020. It went through fifteen years of records at the UC Davis teaching hospital for 35 breeds, and for each breed compared dogs left intact with dogs neutered before six months, at six to eleven months, at one year, and at two to eight years. It counted three joint disorders, hip dysplasia, elbow dysplasia and cranial cruciate ligament tears, and four cancers, lymphoma, mast cell tumour, haemangiosarcoma and osteosarcoma, and in females also mammary cancer, pyometra and urinary incontinence.
The Golden Retriever is the breed that started it. Five percent of intact males had one of the joint disorders; of males neutered before six months, 25 percent did. In females it was 4 percent intact against 18 percent spayed before six months. The cancers went the way nobody expected: 5 percent of intact females, and 11 to 17 percent of spayed females at every spay age, so waiting did not remove it. In males, neutering did not raise the cancer figure.
German Shepherd Dogs: 6 percent of intact males and 5 percent of intact females had a joint disorder, against 19 and 20 percent of those neutered before six months, and the authors suggest waiting until over two years for both sexes. Labrador Retrievers: 6 percent intact, against 13 percent in males neutered before six months and 11 to 12 percent in females spayed in the first year; the suggested guideline is six months or later for males and a year or later for females.
Then the small breeds, where the story stops. No small breed showed an increase in joint disorders with neutering at any age. In Chihuahuas, over a thousand of them, joint disorders were close to nil and cancers under 5 percent whether neutered or not, and the Cavalier, the Maltese, the Pomeranian, the Pug, the Dachshund, the Yorkshire Terrier and the Toy Poodle were the same. Only two small breeds showed a cancer increase: Boston Terrier males neutered in the first year, and Shih Tzu females spayed at a year.
The same group then did mixed-breed dogs, by weight, over twenty years of records. Under 43 pounds, 20 kilograms, there was no increase in joint disorders or cancers at any neutering age. From 43 pounds up, dogs neutered before a year had up to three times the joint disorders of intact dogs, and there was no cancer increase in any weight class. Their guideline: a mixed breed expected to reach 43 to 86 pounds waits until twelve months or beyond, and from 87 pounds, males wait until two years and females until one.
Two things to hold these numbers against. They are records from one university hospital that takes referrals, so the dogs in them are not a random sample of American dogs, and the percentages are of the dogs that hospital saw. And they are associations, not a trial. The authors call their table guidelines, to be weighed with the vet against the reasons for neutering in the first place, and their own summary is the sentence below.
The authors' suggested guideline for each of the 35 breeds, by sex. Where they wrote choice, meaning nothing in their data argued for waiting, this table says any age. The paper is open access and linked at the bottom; take the paper to the vet, not this table.
| Breed | Males | Females |
|---|---|---|
| Australian Cattle Dog | any age | 6 months or later |
| Australian Shepherd | any age | any age, watching for the cancers |
| Beagle | a year or later | any age |
| Bernese Mountain Dog | well beyond 2 years | any age |
| Border Collie | a year or later | a year or later |
| Boston Terrier | a year or later | any age |
| Boxer | 2 years or later | 2 years or later |
| Bulldog | any age | any age |
| Cavalier King Charles Spaniel | any age | any age |
| Chihuahua | any age | any age |
| Cocker Spaniel | 6 months or later | 2 years or later |
| Collie | any age | a year or later |
| Corgi | 6 months or later | any age |
| Dachshund | any age | any age |
| Doberman Pinscher | by a year, or leave intact | 2 years or later |
| English Springer Spaniel | any age | a year or later |
| German Shepherd Dog | 2 years or later | 2 years or later |
| Golden Retriever | a year or later | a year or later, or leave intact; the cancer increase came at every spay age |
| Great Dane | any age; a giant, so the authors suggest well beyond a year | any age; a giant, so the authors suggest well beyond a year |
| Irish Wolfhound | 2 years or later | any age; the authors suggest well beyond a year |
| Jack Russell Terrier | any age | any age |
| Labrador Retriever | 6 months or later | a year or later |
| Maltese | any age | any age |
| Miniature Schnauzer | any age | any age |
| Pomeranian | any age | any age |
| Poodle, Toy | any age | any age |
| Poodle, Miniature | a year or later | any age |
| Poodle, Standard | 2 years or later | any age |
| Pug | any age | any age |
| Rottweiler | a year or later | 6 months or later |
| Saint Bernard | any age | 6 months or later; the authors suggest well beyond a year |
| Shetland Sheepdog | any age | 2 years or later |
| Shih Tzu | any age | 2 years or later |
| West Highland White Terrier | any age | a year or later |
| Yorkshire Terrier | any age | any age |
Pyometra is an infection of the womb that comes on in the weeks after a heat, and it is treated by emergency surgery. In the Swedish insurance data, where most dogs are left intact, 23 to 24 percent of females had it by ten years of age. Spaying removes the organ that gets it.
About half of mammary tumours in dogs are malignant, and Merck's wording for a dog spayed before her first heat is a very low incidence. The exact figures that get repeated, half a percent before the first heat, 8 percent after one and 26 percent after two, come from a single 1969 study, and a 2012 systematic review judged that evidence weak. Treat the direction as solid and the decimals as not. For cats the numbers are above, and they are strong.
Neutering removes the testicles, so testicular cancer with them, and it lowers the chance of the enlarged prostate of the older intact dog. In cats it stops the spraying in most, and in both species it stops the roaming after a female in heat, which is how a good many dogs meet a car. And it stops the litter, which is the reason the shelters campaign for it.
The joint disorders and the cancers in the breeds above, when a big dog is done early. That is the whole reason the dog guidelines changed, and it does not apply to small dogs or to cats.
Weight. A spayed or neutered animal puts it on more easily, and the AVMA lists weight gain among the risks; the bowl has to change with the operation.
Urinary incontinence in spayed females, the leak on the bed while she sleeps. It is on the AVMA's list, and in the UC Davis records it was a breed thing: 25 percent of Doberman females spayed before six months, 33 percent of Shetland Sheepdogs spayed at a year, and in most breeds a few percent or none.
The anaesthetic. The operation is routine, and the ASPCA notes only a slightly higher risk of complications in older, overweight or unwell animals, which is one more argument for doing it at the age the guideline says rather than later.
The cruciate ligament tear counted in the study is the one on Petipedia's cruciate ligament page, and the breed pages carry the study's guideline for the 35 breeds it covered.