For decades, spaying and neutering dogs was often treated as a fairly simple choice: if a dog was not going to be used for breeding, the ovaries or testicles were removed. This prevents unwanted litters and reduces the risk of several diseases of the reproductive system.<\/p>\n
For me, the question has always been more nuanced, and fortunately the scientific literature of recent years has been moving in the same direction.<\/p>\n
The WSAVA Guidelines for the Control of Reproduction in Dogs and Cats, published in 2024, address conventional methods as well as hormone-sparing and non-surgical options, together with their potential long-term benefits and drawbacks. In 2025, WSAVA published an erratum clarifying, among other things, that there had not been full consensus among WSAVA committees and member associations on the recommendations in the ethics section. The guidelines should therefore not be treated as a globally unanimous command, but as an important modern framework for decision-making.<\/p>\n
In 2026, the BVA and BSAVA also updated their recommendations. Spaying female dogs is generally recommended, but depending on breed and physical maturity it is usually advised between 12 and 23 months of age. Neutering male dogs is recommended on a case-by-case basis and, if neutering is chosen, it should not be performed before breed-appropriate physical maturity without a compelling reason.<\/p>\n
This does not mean that spaying or neutering has suddenly become bad. It means that the question is no longer simply, \u201cShould we neuter?\u201d Better questions are: Should we? When? How? And first of all: which dog are we actually talking about?<\/p>\n
A small six-kilogram mixed-breed female, a forty-kilogram male German Shepherd and a female Golden Retriever do not carry the same disease risks and may not experience the same benefits or harms from the same operation. UC Davis studies have shown substantial differences between both breeds and sexes. In the mixed-breed study, adult body size also emerged as particularly important. There is therefore no single ideal age for spaying or neutering every dog.<\/p>\n
In everyday language, sterilization is often used for any procedure after which a dog can no longer reproduce. Biologically, however, there is an important difference depending on whether the organs that produce sex hormones are also removed.<\/p>\n
Gonadectomy means surgical removal of the gonads, namely the testicles in males and the ovaries in females. Conventional surgical neutering and spaying are therefore forms of gonadectomy.<\/p>\n
In a conventional male castration, the testicles are removed. In a female ovariectomy, the ovaries are removed. In an ovariohysterectomy, the ovaries and uterus are removed together. The dog becomes infertile and the hormonal function of the testicles or ovaries is also lost.<\/p>\n
Hormone-sparing options also exist. A male dog can undergo a vasectomy: he cannot sire offspring, but the testicles and their hormone production remain. A female dog can undergo a hysterectomy, in which the uterus is removed while the ovaries are retained. She cannot become pregnant, but ovarian hormonal function remains.<\/p>\n
There is another option as well: temporary hormonal, or so-called chemical, castration. These methods are not hormonally equivalent to one another.<\/p>\n
The testicles and ovaries are not only reproductive organs. The sex hormones they produce are involved in physiology throughout the body and influence, among other things, musculoskeletal development, metabolism, immune function, and processes related to the brain and behaviour. Gonadectomy therefore changes not only a dog’s fertility, but the hormonal environment of the whole body.<\/p>\n
In addition to sexual behaviour and libido, testosterone influences muscle and skeletal development, body composition and a number of behavioural processes. Testosterone also plays a role in immune regulation. After castration, testosterone produced by the testicles falls to very low levels, but this does not mean that all subsequent health or behavioural changes are caused by testosterone deficiency alone.<\/p>\n
Oestrogens, in addition to their role in the female dog’s reproductive cycle, are involved in skeletal development and bone metabolism and also influence metabolism and immune function. Changes in bone tissue have been described after ovariectomy in dogs, but the available studies do not show that spayed dogs develop osteoporosis comparable to postmenopausal osteoporosis in humans. This is a good example of why human and canine physiology cannot be equated one-to-one.<\/p>\n
Progesterone is an important hormone in the female dog’s oestrous cycle. It predominates during dioestrus regardless of whether the dog is pregnant and influences the uterus, mammary glands and pregnancy-related physiology. The canine reproductive cycle differs markedly from the human menstrual cycle.<\/p>\n
All of these hormones are present in both sexes, but in different amounts and from different sources. It is therefore important not to think of them only as \u201creproductive hormones\u201d. After gonadectomy, the entire feedback system between the hypothalamus, pituitary gland and gonads changes, leading, among other things, to persistently elevated LH levels. We return to this mechanism below.<\/p>\n
Observational and population studies have associated gonadectomy with changes in the risk of several diseases and behavioural outcomes, including some orthopaedic diseases, immune-mediated diseases, certain cancers and behavioural problems. These associations do not automatically mean that the absence of one particular sex hormone causes these diseases.<\/p>\n
If a study says that the risk of a disease doubled, this does not automatically mean a large absolute risk. If the baseline risk is 0.1%, doubling it means 0.2%. If the baseline risk is 10%, doubling it means 20%. So saying \u201cthe risk tripled\u201d is not enough. Owners need to know what the starting risk actually was.<\/p>\n
For decades, a claim has been repeated that a female dog spayed before her first heat has a 0.5% risk of mammary tumours, after the first heat 8%, and after the second heat 26%. These figures are easy to read as absolute probabilities of developing the disease. That is not what they mean. These historical numbers describe relative risk compared with intact females.<\/p>\n
The evidence base itself is also less certain than such precise figures suggest. A 2012 systematic review found only 13 eligible studies among more than 11,000 search results. Nine had a high risk of bias and four a moderate risk. A new systematic review in 2025 also identified 13 eligible studies, and 6 of them found no evidence that early spaying had a protective effect. This does not prove that there is no protective effect; it means the old 0.5, 8 and 26 percent model should not be presented as a universal biological law.<\/p>\n
In a Swedish population study of more than 80,000 insured female dogs, a mammary-tumour-related insurance claim had occurred in about 1% by six years of age, 6% by eight years and 13% by ten years. Breed differences were substantial. Mammary-tumour-related mortality was much lower, and the overall case fatality among diagnosed cases in this dataset was about 6%.<\/p>\n
At least three separate questions therefore need to be kept apart:<\/p>\n
These are not the same cancer risk.<\/p>\n
The statement \u201cspaying reduces cancer risk\u201d is also too broad. Gonadectomy naturally eliminates the possibility of tumours arising in the organ that has been removed. At the same time, studies have found associations between gonadectomy and other tumours, including osteosarcoma, lymphoma and hemangiosarcoma. These associations are not the same across all breeds, sexes or ages at surgery.<\/p>\n
A much better question is: which tumour risk does this procedure reduce in this particular dog, which risk might it increase, and how large was the absolute baseline risk of that tumour in the first place?<\/p>\n
The effects of removing the sex organs do not end with the loss of testosterone or ovarian hormones. In an intact dog, a feedback system operates between the hypothalamus, pituitary gland and gonads. When the testicles or ovaries are removed, an important part of the negative feedback disappears and LH remains persistently elevated in gonadectomised dogs. This is a known physiological consequence.<\/p>\n
LH receptors have also been found in tissues outside the reproductive system. In some tissues, a biological response to LH stimulation has been demonstrated. That still does not automatically mean that high LH causes cruciate ligament disease, hypothyroidism, lymphoma, hemangiosarcoma or behavioural problems in a living dog.<\/p>\n
Integrative veterinarian Peter Dobias presents chronically elevated LH more strongly as a central mechanism that could connect several health problems observed after gonadectomy. Michelle Kutzler’s research provides biological support for several links in this hypothesis, but the full causal chain has not yet been established. It is therefore unfair to dismiss the LH hypothesis simply as \u201cunproven\u201d, but equally unfair to write as though the causal relationship had already been proven.<\/p>\n
UC Davis studies show substantial breed- and sex-specific differences. In the mixed-breed study, dogs under 20 kg did not show a significant increase in joint-disorder risk associated with early gonadectomy. In dogs weighing at least 20 kg, however, gonadectomy before one year of age was associated with higher risk; in some groups, joint disorders occurred at roughly three times the rate seen in intact dogs.<\/p>\n
In a meta-analysis of 24 studies, the odds ratio for cranial cruciate ligament disease was 2.29 in gonadectomised females and 2.12 in males compared with intact dogs. The certainty of evidence was rated moderate. However, all included studies were observational. The association is therefore strong enough to take into account when making decisions, but it does not show that gonadectomy alone causes cruciate ligament disease.<\/p>\n
After gonadectomy, energy requirements, appetite and body composition may change. The same amount of food that maintained a dog’s weight before surgery may no longer be appropriate afterwards. Weight gain is not inevitable, but after surgery it is sensible to monitor body condition, muscle mass and energy intake deliberately. This is especially important in larger dogs, because excess weight further increases mechanical load on the joints.<\/p>\n
This issue mainly concerns female dogs. In an RVC VetCompass study based on data from more than 30,000 female dogs, dogs spayed at 7 to 12 months had about a 20% lower risk of early-onset urinary incontinence than dogs spayed at 3 to 6 months. Higher body weight also increased risk, and some breeds were at greater risk than others. This does not mean that later spaying prevents urinary incontinence altogether; it shows that timing can also influence the risk.<\/p>\n
Neutering can affect behaviour that is directly linked to sex hormones. In males, seeking females, roaming, urine marking and mating behaviour may decrease. In some dogs, male-male competitive aggression may also decrease. This does not mean that testosterone is simply an \u201caggression hormone\u201d.<\/p>\n
Fear aggression, anxiety, territorial behaviour, pain-related aggression and sexual competition between males are different problems. Before an irreversible operation, the question should therefore be: which behaviour are we actually trying to change?<\/p>\n
Chemical castration in dogs can be achieved, among other methods, with a deslorelin implant. Deslorelin is a GnRH agonist. Initially it stimulates the reproductive hormone axis, but with continuous action LH and FSH, and in males testosterone, are subsequently suppressed.<\/p>\n
The initial stimulation phase, or flare-up, is important. Before the hormonal axis is suppressed, sex-hormone-related activity may temporarily increase. In a small randomised double-blind study published in 2021, sex-hormone-related problem behaviour worsened without flare-up-blocking medication in 56% of dogs according to veterinarians and 66% according to owners. So an owner’s experience that \u201cthe implant made my dog worse at first\u201d can be entirely real.<\/p>\n
One of the most interesting uses of chemical castration is as a reversible trial before surgical castration. If a dog is being considered for castration because of a behavioural problem, temporary hormonal suppression can help assess what actually happens to that individual dog’s behaviour.<\/p>\n
Chemical castration is not hormonally equivalent to vasectomy. A vasectomised male remains hormonally similar to an intact dog, whereas during the action of deslorelin the hormonal axis is suppressed.<\/p>\n
A small double-blind randomised placebo-controlled study published in 2026 included 30 already surgically castrated males. Deslorelin suppressed LH, and the phenomenon being studied, sexual interest from other males towards the castrated dogs, decreased in 11 of 15 dogs receiving deslorelin compared with 5 of 15 in the placebo group. The study needs replication, but it is interesting because it makes the LH hypothesis testable through an intervention.<\/p>\n
The choice does not always have to be only between \u201cleave the dog intact\u201d and \u201cremove the gonads\u201d. Vasectomy and ovary-sparing hysterectomy can prevent reproduction without removing the gonads.<\/p>\n
A 2023 study collected online questionnaire data on more than 6,000 dogs and compared intact dogs, gonadectomised dogs and dogs that had undergone hormone-sparing procedures. The hormone-sparing groups were still small: 58 vasectomised males and 159 females with ovary-sparing surgery. It was also an owner survey rather than a randomised long-term clinical trial.<\/p>\n
A vasectomised male retains his testicles, so risks of testicular disease, testosterone-dependent behaviours and the risk of BPH also remain. With ovary-sparing hysterectomy the ovaries are retained: the female cannot become pregnant and the uterus has been removed, but the hormonal cycle continues and ovarian risks remain. This is not a risk-free option; it is a different risk profile.<\/p>\n
Prostatic tumours are rare in dogs. A protective effect of castration against prostate cancer has not been shown. In one large population study, castrated males had an odds ratio of 2.24 for diagnosis of all prostate cancers compared with intact males. This does not prove that castration causes prostate cancer, but preventing prostate cancer is not an accurate argument for castrating a healthy young male.<\/p>\n
BPH, or benign prostatic hyperplasia, is a completely different condition. It is very common in older intact males, and castration is a highly effective treatment for symptomatic BPH. Prostate enlargement is not prostate cancer.<\/p>\n
Pyometra, or uterine infection, is one of the strongest medical arguments in favour of spaying a female dog. In Swedish data covering more than 260,000 female dogs and over 1,000,000 dog-years, about 19% of females had been diagnosed with pyometra by 10 years of age. The average age at diagnosis was 7 years. In an older Swedish dataset, the annual risk of pyometra in females younger than ten was approximately 2%. Risk varied markedly by breed.<\/p>\n
So it is not accurate to say either \u201call intact females will get pyometra\u201d or \u201cpyometra is rare\u201d. Morbidity and mortality also need to be kept separate. Pyometra can be life-threatening and requires prompt treatment, but a 19% risk of developing the disease does not mean that 19% of females die from it.<\/p>\n
For a female dog, the question may be less \u201cshould we spay?\u201d and more \u201cwhen, and by which method?\u201d<\/p>\n
UC Davis studies have perhaps shown most clearly why one universal age does not work. For some breeds and sexes, earlier gonadectomy was not associated with a significant increase in the diseases studied. In some, a clear signal emerged when surgery was performed before one year of age. In some large breeds, the data suggest waiting considerably longer. And for some specific breed-sex combinations, the disease data studied do not support gonadectomy at all.<\/p>\n
These tables are not spay-and-neuter calendars. If a study uses 12 months as a practical reference point, that does not mean an 11-month-and-29-day-old dog is hormonally immature and suddenly ready the next morning. A dog’s biology does not read Excel.<\/p>\n
\u201cWait until physical maturity\u201d does not mean \u201conce physical maturity is reached, the dog must immediately be neutered.\u201d Those are two different questions.<\/p>\n
The table is not a scoring system. Its purpose is to help ask: \u201cI am worried about this particular disease. How large is my dog’s actual risk, and what does the chosen procedure do to that risk?\u201d<\/p>\n