Why sterilise your cat or dog?

The short version

What the guidelines say, animal by animal

This is the position of the WSAVA guidelines and the AAHA canine guidelines. Where they disagree, we say so.

• Female cats

  Spaying is recommended, ideally before puberty. It prevents pregnancy and uterine disease, and spaying before six months was linked to a 91% lower risk of mammary carcinoma.

• Male cats

  Castration is recommended. It reduces urine spraying, roaming and fighting, and fighting is the main route by which FIV spreads.

• When for cats

  Around four months, before a kitten can become pregnant. WSAVA says healthy kittens can be sterilised as early as three to four months.

• Small and medium dogs

  Sterilisation remains a reasonable option. AAHA suggests about six months for dogs expected to weigh under about 20 kg as adults.

• Large-breed dogs

  Wait until growth stops, usually 9 to 15 months. In some large breeds, neutering before one year was linked to two to four times more joint disorders.

• Female dogs

  Here the guidelines disagree. WSAVA says a healthy bitch may stay intact if monitored, and if spayed, ideally after the first or second heat. AAHA advises spaying small breeds before the first heat.

• Male dogs

  WSAVA suggests keeping them intact or considering vasectomy if the owner is responsible. Castration is required for retained testicles and strongly advised for recurring prostate enlargement.

• Every sterilised pet

  Energy needs fall by roughly a quarter to a third. Feed less from the start, and weigh regularly.

The basics

What sterilisation actually means

In females, spaying removes the ovaries (ovariectomy) or the ovaries and uterus (ovariohysterectomy). In males, castration removes the testicles. Both stop reproduction and remove the main source of sex hormones, and most of the benefits and most of the costs come from losing those hormones.

There are also options that stop reproduction but keep the hormones. The WSAVA guidelines describe vasectomy in males and ovary-sparing hysterectomy in females, as well as non-surgical methods such as hormone implants. A deslorelin implant suppresses fertility in male dogs for at least six or twelve months depending on the dose, and it wears off, so owners can see how a dog behaves without testosterone before deciding on surgery.

Cats

For cats, the guidelines agree

There is little debate about cats. The WSAVA executive summary says spaying is generally recommended for female cats because intact females are hard to live with at home, and castration is generally recommended for males because it removes unwanted behaviour. Healthy, vaccinated kittens of average-sized breeds, in good body condition, can be sterilised from about three to four months.

The timing matters because cats mature early. A kitten can become pregnant at around four months, which is why the UK Cat Kind group and International Cat Care recommend neutering pet cats at four months rather than the older six-month norm. The British Veterinary Association, the British Small Animal Veterinary Association and the Federation of Veterinarians of Europe support neutering from 16 weeks. The FVE treats neutering a cat after four months as late.

Cats

Mammary cancer: the strongest number in this article

In cats, mammary tumours are usually malignant: 80–90% of them, according to a WSAVA Congress review. In a 2005 case-control study of 204 cats with mammary carcinoma and 200 controls, intact cats were overrepresented among the cancer cases. Cats spayed before six months had a 91% lower risk of mammary carcinoma than intact cats, and cats spayed before one year an 86% lower risk. Spaying between one and two years cut the risk by only about 11%, and after two years the benefit largely disappeared.

Having kittens didn’t change the risk. In the same study, the number of litters had no effect on mammary carcinoma, so a queen gains nothing by having “one litter first”.

This is still one study. A later evidence review rated the support for spaying before one year as weak, because only this study gave enough age data to test it. The direction of the effect is consistent, and it is the main medical reason vets push for early spaying in cats.

Cats

Spraying, roaming and the fights that spread FIV

An intact tomcat marks his territory with strong-smelling urine, roams widely and fights other males. Castration reduces spraying in about 90% of cats, whatever their age or how long the habit has lasted, according to a review of the behavioural literature. VCA reports a similar reduction in roaming.

The fights matter for health, not only for the furniture. Feline immunodeficiency virus (FIV) spreads mainly through deep bite wounds, and Cornell’s Feline Health Center names intact males with outdoor access, who are likely to fight, as the cats at greatest risk. Neutering won’t make an outdoor cat safe, but it reduces the roaming and fighting that expose him.

Castration doesn’t solve everything. After neutering before puberty, urine marking was still seen in about 10% of males and 5% of females, according to data cited in the WSAVA guidelines.

Female dogs

Pyometra: the reason most vets bring up first

Pyometra is a bacterial infection of the uterus that can kill a bitch if it isn’t treated, usually with emergency surgery. The best data come from Sweden, where spaying was traditionally done mainly for medical reasons and only about 7% of bitches were spayed, so the natural rate can be seen. In Swedish insurance data, 23–24% of intact bitches had pyometra by ten years of age, ranging from 10% to 54% depending on breed. A later study of more than 260,000 insured bitches put the figure at 19% by age ten, with a mean age at diagnosis of seven years.

Removing the uterus prevents pyometra. That is why even the WSAVA guidelines, which are cautious about spaying dogs, say an ovariohysterectomy should be done when there is uterine disease. Cats get pyometra too. In Swedish insurance data, about 2.2% of female cats had it by 13 years of age (Hagman et al., 2014). That figure counts spayed and intact cats together, because the database did not reliably record spaying, so the figure for intact cats alone would be higher.

Female dogs

Mammary tumours: common, but the famous numbers are weak

Mammary tumours are common in intact bitches. In the same Swedish insurance data, 13% had them by ten years of age. Most guidelines say spaying lowers the risk. AAHA says spaying before the first heat minimises it, and that the risk rises with each heat up to about two and a half years of age.

The evidence behind that is thinner than it sounds. A 2012 systematic review found 13 relevant studies, judged 9 to be at high risk of bias, and concluded the evidence was weak and “not a sound basis for firm recommendations”. The often-quoted percentages for spaying before the first, second or later heat come largely from one study from 1969, in which only one dog had been spayed before her first heat.

Most vets still read the evidence as protective. The size of the effect, and the exact cut-off by heat cycle, are less certain than the numbers you may have seen.

Female dogs

Urinary incontinence: the cost on the other side

Spaying is linked to a higher risk of urinary incontinence, usually from a weaker urethral sphincter. In a UK cohort that followed bitches from 12 weeks of age, 5.2% of spayed bitches and 1.0% of intact bitches became incontinent within five years. Heavier dogs are affected more. In a US study of bitches over 25 kg spayed in their first year, every month of delay lowered the risk.

Incontinence can usually be treated with medication, but it is lifelong. It is one reason the guidelines now ask for more thought about when to spay large female dogs.

Male dogs

Castration in dogs: fewer reasons than people think

Castration prevents testicular disease and benign prostatic hyperplasia (BPH), the age-related enlargement of the prostate. BPH is very common: it affects over 80% of intact male dogs over five, though most never show signs. Castration is the definitive treatment, and the WSAVA guidelines strongly recommend castration or hormonal treatment for dogs with recurring signs of BPH. When a testicle hasn’t descended, castration is obligatory, because a retained testicle can become a hormone-secreting tumour.

Beyond that, the case for routinely castrating owned male dogs is weaker than many owners assume. For male dogs with responsible owners, WSAVA recommends either keeping them intact or considering vasectomy if sterilisation is wanted. Its committee has also warned that castrating older dogs to prevent BPH may not be appropriate, because in some cases late castration may increase the risk of prostatic carcinoma.

Behaviour

Neutering is not a cure for aggression

Castration changes behaviours driven by testosterone. In a study of 57 male dogs, roaming, urine marking and mounting were the behaviours most likely to change. Aggression towards people or dogs in the household fell in about a quarter of dogs.

Larger studies are less encouraging. A 2018 study using a validated behaviour questionnaire on thousands of dogs found no link between neutering, or age at neutering, and aggression towards familiar people or dogs. Dogs neutered at 7 to 12 months were 26% more likely to show moderate or severe aggression towards strangers.

If a dog’s behaviour is the main concern, a hormone implant can be used as a trial before anything permanent is done, alongside behavioural advice.

Large dogs

Why breed and size change the answer

The biggest shift in the evidence comes from the University of California, Davis. Its earlier work found that in Golden Retrievers, Labrador Retrievers and German Shepherds, neutering before one year was linked to two to four times more joint disorders (hip dysplasia, elbow dysplasia, cruciate ligament tears) than in intact dogs. Its 2020 study extended the analysis to 35 breeds.

The picture varies a great deal by breed. Small breeds showed no increase in joint disorders with neutering, and only two small breeds, the Boston Terrier and the Shih Tzu, showed a significant increase in cancers. The two giant breeds studied, the Great Dane and the Irish Wolfhound, showed no increase in joint disorders at any age of neutering. In most breeds, the age of neutering didn’t affect the risk. For dogs without a breed, a companion study grouped mixed-breed dogs into five adult-weight categories.

That is why AAHA splits its advice by expected adult weight. Dogs expected to weigh under about 20 kg (45 lb): castrate at about six months, spay before the first heat (five to six months). Larger dogs: castrate once growth stops, usually 9 to 15 months. For large females, the AAHA timing (5 to 15 months) leaves room for judgement, trading the mammary benefit of early spaying against the joint and incontinence risks.

Lifespan

Do sterilised pets live longer?

On average, yes, though the reasons aren’t fully clear. In more than 40,000 dogs that died at US veterinary teaching hospitals, sterilised dogs lived longer: 13.8% longer for males and 26.3% for females. They were less likely to die of infection or trauma and more likely to die of cancer or immune-mediated disease. The authors had no data on the age at which dogs were sterilised, and sterilised dogs may also have had more veterinary care.

In cats the gap is larger. In Banfield’s 2013 report on 460,000 cats, neutered males lived on average 62% longer than intact males and spayed females 39% longer than intact females. These are observational figures: an intact cat is also more likely to roam, fight and be hit by a car, and sterilisation may partly be a marker of the care a pet receives.

The trade-offs

What sterilisation can cost

Weight. This is the most common cost and the easiest to prevent. After spaying, adult cats needed about 25% less energy than the standard recommendation to keep their weight, and the Purina Institute advises cutting a pet’s daily calories by around 30% after surgery, then adjusting to body condition. The surgery doesn’t make a pet fat, but the same bowl of food will.

Disease risks in some dogs. Depending on breed and size: more joint disorders, some cancers, and urinary incontinence in bitches, as described above. In cats, the WSAVA guidelines note a higher risk of lower urinary tract disease in neutered cats, and that neutering before puberty can delay the closure of growth plates, which may contribute to growth-plate fractures.

Anaesthesia. Any surgery carries anaesthetic risk, but in healthy animals it is low. In a UK study of nearly 180,000 dogs and cats, the risk of anaesthesia-related death in healthy patients was about 0.05% in dogs (1 in 1,849) and 0.11% in cats (1 in 895). Around half of the deaths in dogs, and more in cats, happened after the operation, in recovery, so post-operative monitoring matters.

Myths

Three things that aren’t true

“She should have one litter first.” There is no evidence that a litter benefits a cat or a dog. In cats, the number of litters had no effect on mammary cancer risk, while every month of delay past six months reduced the protection spaying gives.

“Neutering will calm him down.” It reduces behaviours driven by sex hormones, such as roaming, marking, mounting and, in cats, fighting. It doesn’t reliably reduce aggression in dogs, and in one large study dogs neutered at 7 to 12 months were more likely to be aggressive towards strangers.

“Sterilised pets always get fat.” Their energy needs fall, so they gain weight on the same diet. Feeding less from the day of surgery prevents most of it.

In practice

What the guidance adds up to

1. Cats: sterilise both sexes, ideally at about four months and before a female’s first heat.
2. Cats that go outside should be sterilised before they first go out.
3. Small and medium dogs: sterilisation at about six months, or before a bitch’s first heat, remains reasonable.
4. Large and giant dogs: if you sterilise, wait until growth is complete, usually 9 to 15 months, and ask about your breed’s data.
5. Female dogs: weigh the risk of pyometra and mammary tumours against incontinence and breed-specific risks. Discuss timing with your vet rather than following a fixed age.
6. Male dogs: there are fewer medical reasons to castrate. Retained testicles and recurring prostate problems are clear ones; behaviour alone usually isn’t.
7. Ask about alternatives: vasectomy, ovary-sparing surgery, or a temporary hormone implant.
8. If you keep a female dog intact, ask your vet which signs of pyometra to watch for, and see them promptly if she seems unwell.
9. After any sterilisation, cut food by about a quarter to a third and weigh your pet regularly.
10. Never let a bitch or queen have a litter “for her health”. There is no health benefit.

Your vet

Questions to take to the appointment

• How big will my dog be as an adult, and when will growth stop?
• Is there breed-specific evidence on neutering age for my dog’s breed?
• If we wait, how do we manage heats and watch for pyometra?
• Would a vasectomy, an ovary-sparing spay or an implant suit my animal?
• How much should I feed after the surgery, and how often should we weigh?

The limits

What the evidence can’t tell you

Almost all of this research is observational. It comes from insurance databases, teaching hospitals and owner questionnaires, mostly in the United States and Sweden. Sterilised and intact pets differ in more than their hormones: in how they live, how much they roam and how much care they get. The breed-by-breed data come largely from one hospital, and many breeds were never studied.

The guidelines themselves disagree. AAHA still recommends sterilising all dogs not intended for breeding; WSAVA says the decision should be made case by case. Both are written by veterinary experts reading the same studies. For cats, the evidence and the guidelines point the same way. For dogs, the right answer depends on sex, size, breed, lifestyle and how well a household can manage an intact animal, which is a conversation for you and your vet.

The source

“Should I spay my dog?”

Lares Pecoris is a veterinary reference tool. It answers from a curated set of official veterinary sources, including guidelines like those cited here, and shows the document behind each answer. When those sources don’t cover a question, it says so. It can help you understand what the guidelines say about your animal’s species, size and breed before you talk to your vet. It is not a veterinarian and does not replace one.

Questions

Frequently asked questions

Sources

1. Romagnoli S, Krekeler N, de Cramer K, Kutzler M, McCarthy R, Schaefer-Somi S. WSAVA guidelines for the control of reproduction in dogs and cats. J Small Anim Pract 2024;65(7):424–559. doi:10.1111/jsap.13724. Executive summary, June 2024.
2. Imrie P. WSAVA wants ‘paradigm shift’ in thinking on reproduction control. Vet Times, 3 June 2024.
3. American Animal Hospital Association. 2019 AAHA Canine Life Stage Guidelines: Spay and neuter timing and Reproductive health.
4. Hart BL, Hart LA, Thigpen AP, Willits NH. Assisting decision-making on age of neutering for 35 breeds of dogs: associated joint disorders, cancers, and urinary incontinence. Front Vet Sci 2020;7:388. doi:10.3389/fvets.2020.00388.
5. Hart BL, Hart LA, Thigpen AP, Willits NH. Assisting decision-making on age of neutering for mixed breed dogs of five weight categories: associated joint disorders and cancers. Front Vet Sci 2020;7:472. doi:10.3389/fvets.2020.00472.
6. University of California, Davis. When should you neuter your dog to avoid health risks?, July 2020.
7. Egenvall A, Hagman R, Bonnett BN, Hedhammar A, Olson P, Lagerstedt AS. Breed risk of pyometra in insured dogs in Sweden. J Vet Intern Med 2001;15(6):530–538.
8. Jitpean S, Hagman R, Ström Holst B, Höglund OV, Pettersson A, Egenvall A. Breed variations in the incidence of pyometra and mammary tumours in Swedish dogs. Reprod Domest Anim 2012;47(Suppl 6):347–350. doi:10.1111/rda.12103.
9. Hagman R. New aspects of canine pyometra: studies on epidemiology and pathogenesis. Doctoral thesis, Swedish University of Agricultural Sciences, 2004.
10. Hagman R, Ström Holst B, Möller L, Egenvall A. Incidence of pyometra in Swedish insured cats. Theriogenology 2014;82:114–120. doi:10.1016/j.theriogenology.2014.03.007.
11. 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;53(6):314–322. doi:10.1111/j.1748-5827.2011.01220.x.
12. The gonadectomy controversy. Veterinary Practice News.
13. Overley B, Shofer FS, Goldschmidt MH, Sherer D, Sorenmo KU. Association between ovariohysterectomy and feline mammary carcinoma. J Vet Intern Med 2005;19(4):560–563.
14. Sorenmo KU. Mammary gland tumors in cats: risk factors, clinical presentation, treatments and outcome. WSAVA World Congress Proceedings, 2011.
15. Improve International Clinical Library. Should we recommend spaying cats for mammary tumour prevention? At what age?
16. Hoffman JM, Creevy KE, Promislow DEL. Reproductive capability is associated with lifespan and cause of death in companion dogs. PLoS One 2013;8(4):e61082. doi:10.1371/journal.pone.0061082.
17. Banfield Pet Hospital. Spaying and neutering may contribute to longer lifespans (State of Pet Health 2013 Report), 8 May 2013.
18. Farhoody P, Mallawaarachchi I, Tarwater PM, Serpell JA, Duffy DL, Zink C. Aggression toward familiar people, strangers, and conspecifics in gonadectomized and intact dogs. Front Vet Sci 2018;5:18. doi:10.3389/fvets.2018.00018.
19. Effects of gonadectomy on behavior of dogs and cats (Proceedings). dvm360, 2011, citing Neilson JC et al., JAVMA 1997;211:180–182, and Hart BL, Cooper L, JAVMA 1984;184:1255–1258.
20. Cornell Feline Health Center. Feline immunodeficiency virus (FIV).
21. VCA Animal Hospitals. Cat neutering and behavior.
22. International Cat Care. Neutering your cat.
23. McDonald J, Clements J. Contrasting practices and opinions of UK-based veterinary surgeons around neutering cats at four months old. Vet Rec 2020. doi:10.1136/vetrec-2020-105887.
24. Cat Kind (formerly the Cat Population Control Group). Neutering kittens at four months: a summary of the evidence.
25. Brodbelt DC, Blissitt KJ, Hammond RA, et al. The risk of death: the Confidential Enquiry into Perioperative Small Animal Fatalities. Vet Anaesth Analg 2008;35(5):365–373. doi:10.1111/j.1467-2995.2008.00397.x.
26. Mitsuhashi Y, Chamberlin AJ, Bigley KE, Bauer JE. Maintenance energy requirement determination of cats after spaying. Br J Nutr 2011;106:S135–S138.
27. Purina Institute. Decreased energy needs in pets after spaying or neutering.
28. Thrusfield MV, Holt PE, Muirhead RH. Acquired urinary incontinence in bitches: its incidence and relationship to neutering practices. J Small Anim Pract 1998;39:559–566.
29. Byron JK, Taylor KH, Phillips GS, Stahl MS. Urethral sphincter mechanism incompetence in 163 neutered female dogs: diagnosis, treatment, and relationship of weight and age at neuter to development of disease. J Vet Intern Med 2017;31(2):442–448.
30. Geddes C. Benign prostatic hyperplasia in dogs. British Veterinary Nursing Association, Veterinary Nursing Journal, August 2022.
31. Driancourt MA, Briggs JR. Gonadotropin-releasing hormone (GnRH) agonist implants for male dog fertility suppression: a review of mode of action, efficacy, safety, and uses. Front Vet Sci 2020;7:483.

All sources accessed 12 November 2026.