Neutering your male dog

much more than just preventing puppies

Before it became a matter of “opinion”, neutering was promoted in cities for public health reasons: control of stray animals, accidents, bites, disease transmission and shelter overcrowding.

Even so, talking about neutering male dogs is not just about reproduction. It is about long-term health, hormonal balance, behaviour, disease prevention and, above all, animal welfare. Although many owners use terms such as castration and vasectomy interchangeably, the truth is that they are not equivalent procedures, nor do they have the same effects on the dog’s body or behaviour.

From an epidemiological point of view, population studies conducted in Europe and North America show that more than 70% of reproductive and prostate problems diagnosed in adult dogs occur in unneutered males. Furthermore, a database of more than 70,000 animals found that neutered dogs lived longer than unneutered dogs (e.g., 9.4 vs. 7.9 years), and in cats the difference was even greater, with neutered males living up to 62% longer than their intact counterparts. Therefore, the choice of technique should not be based solely on avoiding a litter, but on an overall assessment of future medical risks, lifestyle and the animal’s environment.

Neutering: when surgery also acts on hormones ✂️

Surgical castration involves the removal of the testicles, the organs responsible for the production of sperm and testosterone, the main male sex hormone. Medically, this is a bilateral orchiectomy, a widely standardised procedure in veterinary medicine, with a complication rate of less than 2% when performed on healthy animals using current anaesthetic protocols.

Testosterone plays a central role not only in fertility, but also in modulating behaviour, intraspecific aggression, stress response and the maintenance of certain tissues. After castration, testosterone levels decline progressively, reaching minimum values approximately 4 to 6 weeks after surgery. During this period, it is common for the dog to show gradual changes in behaviour rather than sudden transformations.

From a practical point of view, this interval is ideal for reinforcing stable routines, controlled exercise and positive learning, as several studies in canine ethology indicate that hormonal reduction improves concentration and decreases impulsivity, facilitating educational work.

Testosterone does not disappear suddenly after castration

An endocrinological study showed that although testicular production ceases after castration, low testosterone levels may persist for several weeks due to extragonadal production (adrenal glands). This explains why some sexual behaviours do not disappear immediately after surgery.

Source: Root Kustritz, M. V. – Journal of Veterinary Internal Medicine

Benefits VS disadvantages of castration in male dogs ✅

Real benefits of castration

Prevention of the risk of testicular tumours (risk eliminated if castrated in time)

The medical benefits of neutering are widely documented. Testicular removal eliminates 100% of the risk of testicular tumours, which have a prevalence of 15-27% in unneutered dogs over 7 years of age, depending on breed and body size.

Testicular tumours: common but not very visible

A retrospective study in European veterinary hospitals showed that up to 27% of geriatric intact male dogs develop testicular tumours, many of which are detected incidentally during routine examinations.

Source: Hayes, H. M. – Journal of Small Animal Practice

From an oncological point of view, castration eliminates the possibility of developing testicular tumours, provided it is performed before the tumour exists. This is particularly relevant in adult dogs, because testicular neoplasms increase with age and are often detected late (e.g. during routine check-ups or when scrotal asymmetry is noticed). Furthermore, in dogs with cryptorchidism (undescended testicles), the risk of neoplastic transformation is clearly higher, so the surgical recommendation is usually primarily medical and preventive, not reproductive.

In educational terms: by removing the testicular tissue, the “target organ” where these tumours originate is eliminated. This also prevents associated problems such as testicular torsion, inflammation (orchitis/epididymitis) or chronic local pain, which in older dogs can appear as non-specific symptoms (persistent licking, irritability, refusal to walk or sit).

Cryptorchidism: the risk of testicular tumours is not “a little” higher, it can multiply.

A classic study estimated a relative risk of ~9 for developing testicular tumours in cryptorchid dogs compared to normal dogs. This anecdote fits in very well when explaining why, in a male with an undescended testicle, the recommendation for surgery is usually much more “medical” than “reproductive”. Source: PubMed (Hayes Jr., 1985).

https://pubmed.ncbi.nlm.nih.gov/2863879/

Decreased or eliminated risk of perianal tumours (perianal adenomas)

Circumanal adenomas (also called perianal/circumanal gland tumours) are a classic feature of the intact male: they usually present as nodules around the anus, sometimes ulcerated or bleeding, and have a very important characteristic: in a large proportion of cases they are hormone-dependent, i.e. they grow stimulated by androgens such as testosterone.

Therefore, castration not only prevents their appearance in many predisposed dogs, but can also form part of the treatment: regression after castration has been reported in a very high percentage of benign adenomas (in some studies, up to ~95%), although any perianal mass must always be confirmed by cytology/biopsy so as not to confuse it with malignant lesions (e.g. adenocarcinoma). 

Put simply for the owner: if the lesion is “fed” hormonally, by drastically reducing this stimulation, the tumour usually slows down or even shrinks. In practice, the best strategy is often to combine local removal + castration, because it reduces recurrences and attacks the underlying biological factor.

Prostate problems: the great preventive (and therapeutic) advantage

In male dogs, the prostate is one of the organs most influenced by testosterone. Benign prostatic hyperplasia (BPH) is extremely common with age: recent publications estimate its presence in around 80% of intact males >5 years old and over 95% in intact males >9 years old, according to the diagnostic criteria used.

The important thing is that castration is the most effective method for reducing prostate size: a decrease in volume of up to ~70% has been reported, with changes that can begin to be observed within a few weeks. In fact, ultrasound research has shown that prostate volume can be reduced by 40–70% in the first 3–4 weeks after castration in dogs with benign prostatic hyperplasia, even without additional drug treatment. Clinically, this translates into real improvements when symptoms were present: less tenesmus (straining during defecation), less urethral dribbling, less discomfort, fewer associated urinary tract infections, etc.

An important (and very professional) nuance: canine prostate cancer is rare and its relationship to castration is a debated issue; castration is clearly beneficial for BPH and other benign/proliferative androgen-dependent conditions, but in prostate neoplasia the approach is different and requires individualisation.

Behaviour and aggression: what is known with certainty (and what is not)

Castration-induced changes usually result in a more emotionally stable animal, with better quality of rest and greater ability to adapt to the family environment. Behaviourally, longitudinal studies have shown that castration reduces:

• excessive mounting in approximately 60–70% of cases,
• urine marking by around 50–60%,
• and certain aggressive behaviours related to sexual competition by up to 40%, especially when performed before such behaviours become deeply ingrained.

Did you know?

Males can detect a female in heat from more than 1 km away (2 km diameter) thanks to their extremely sensitive sense of smell, which explains why many dogs ‘run away’ when they smell a female in heat nearby. 

A study published in ScienceDirect indicated that neutered dogs are significantly less likely to roam or run away, which can reduce accidents and fights between animals, although this effect is more pronounced in dogs than in cats. 

Here we must be very honest: neutering is not a “universal treatment” for aggression. Aggressive behaviour can have multiple causes (fear, pain, territoriality, resource protection, learning, lack of socialisation, anxiety, etc.), and in these cases neutering may not improve the situation and, in some cases, may not even produce any significant changes. Reviews emphasise that the results are heterogeneous and depend on the type of aggression, age, learning history and context. One scientific review found that neutering reduces reproductive behaviours such as urine marking, mounting and wandering, but it can also increase certain aggressive or fearful behaviours in some individuals, reflecting that the behavioural effect is not universal. In this regard, a study of 8,981 dogs analysed 23 behaviours and found that the differences between intact and neutered dogs were not always related to sex hormones, and that some persistent behaviours may be due more to previous experience than to hormonal changes. 

What is better supported is that neutering can reduce intraspecific aggression linked to sexual competition, especially male-male (e.g., status conflicts, rivalry over females in heat, or selective reactivity towards other males), and can also decrease associated behaviours such as escapism, marking, and mounting in a significant proportion of dogs.

Therefore, the most sound clinical approach is usually:

• If there are male-male conflicts with a clear sexual/competitive component, castration can be a useful aid.
• If aggression appears to be motivated by fear, insecurity, or pain, the priority is medical diagnosis (ruling out pain, endocrine, neurological) and a behaviour modification plan; castration, if decided upon, should be considered a secondary measure, not “the solution”.

In summary: neutering has frequent behavioural benefits, but its effect on aggression is selective; the evidence is more consistent in inter-male aggression and less consistent (or controversial) in other categories.

Practical scientific note: behaviour ≠ 100% hormone.

If a dog has learned that barking/fighting/running away “works” for them, neutering may reduce their sex drive, but it does not erase what they have learned. Therefore, when there is aggression or reactivity, the combination that works best is usually: medical examination + sterilisation if necessary + behavioural modification plan (and, if necessary, an ethologist). This idea adds value without repeating the castration/vasectomy comparison as it stands. 

Are there any drawbacks to neutering? Yes, and it is important to be aware of them ⚠️

After neutering, basal metabolism can decrease by 20-30%, which explains why neutered dogs are at greater risk of becoming overweight if their diet is not adjusted. Nutritional studies indicate that up to 50% of neutered dogs gain weight during the first year when their food intake and level of physical activity remain unchanged.

This risk is significantly reduced when simple measures are taken: controlling calorie intake, specific diets for neutered dogs, and regular exercise. As for muscle mass, the loss is usually slight and gradual, and is mainly observed in dogs with insufficient physical activity.

From an anaesthetic point of view, the mortality rate associated with general anaesthesia in healthy dogs is less than 0.05%, and decreases even further when a complete preoperative evaluation with blood tests and advanced intraoperative monitoring is performed.

Benefits vs. drawbacks of vasectomy in male dogs 🔬

Vasectomy: sterilisation without affecting hormones 

Vasectomy involves cutting or blocking the vas deferens, preventing sperm from being released with ejaculate. The dog maintains normal testosterone levels, so its hormonal physiology and sexual behaviour remain virtually unchanged. A comparative study confirmed that testosterone levels in vasectomised males do not differ from those in intact males, which explains the persistence of libido and sexual behaviour after vasectomy.

From a biological point of view, this means that the dog will continue to display libido, mounting and marking behaviours, as well as interest in females in heat. Comparative studies indicate that there are no significant differences in behaviour or prostate pathology between intact males and vasectomised males.

Important limitations of vasectomy ❌

Vasectomy does not reduce the risk of hormone-dependent diseases, testicular tumours or prostate pathologies. Nor does it modify problematic sexual behaviours. Furthermore, its availability is limited and its use in clinical veterinary medicine is rare, accounting for less than 5% of sterilisations performed on male dogs in most European countries.

Therefore, although it is effective in preventing fertilisation, its impact on the overall health of the animal is clearly inferior to that of castration.

So… which option is best for your dog? ⚖️

If the sole objective is to prevent breeding, both techniques fulfil their function. However, when seeking to prevent common diseases, reduce future medical risks and improve certain behavioural aspects, castration offers clinical and preventive benefits that have been widely demonstrated in scientific literature.

Assessing the dog’s age, breed, environment and temperament together with your vet will allow you to choose the most appropriate option and plan a personalised follow-up, thus ensuring a better quality of life and life expectancy ❤️.

🩺 Optimised comparison table: Castration vs Vasectomy in male dogs

Sources and recommended reading

Life expectancy: neutered animals live longer, but the causes of death change.

In a large-scale analysis of medical records, neutered dogs showed greater longevity and a lower probability of dying from infections, while the contribution of causes such as cancer or immune-mediated diseases increased proportionally (this does not mean “worse”, but rather a change in the risk profile due to living longer). Source: PLOS ONE (Hoffman et al., 2013). https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0061082

“Age” matters: in Golden Retrievers, early neutering was associated with more joint problems.

In Golden Retrievers, neutering before one year of age was associated with a striking increase in hip dysplasia in males, and the behaviour of certain cancers and orthopaedic injuries was also analysed according to age at sterilisation. This is a useful anecdote to explain why the recommendation may be individualised by breed/size. Source: UC Davis News on the PLOS ONE study (Torres de la Riva et al., 2013). https://www.ucdavis.edu/news/golden-retriever-study-suggests-neutering-affects-dog-health

(Original article) https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0055937

Cryptorchidism: the risk of testicular tumours is not “a little” higher, it can multiply.

A classic study estimated a relative risk of ~9 for developing testicular tumours in cryptorchid dogs compared to normal dogs. This anecdote fits very well when explaining why, in a male with an undescended testicle, the recommendation for surgery is usually much more “medical” than “reproductive”. Source: PubMed (Hayes Jr., 1985). https://pubmed.ncbi.nlm.nih.gov/2863879/

Cats: neutering can reduce marking/roaming/fighting… and this was observed quickly.

In a classic study (yes, one of those cited time and time again in veterinary ethology), after neutering male cats, a marked reduction in fighting, escaping and urine marking was reported, with improvement in many cases occurring early on. Very useful as “scientific data” in a box on feline behaviour. Source: JAVMA (Hart, 1973) – PDF. https://avmajournals.avma.org/view/journals/javma/163/3/javma.1973.163.03.290.pdf

Dogs: there is no “universal age” for all dogs; there are breeds where the risk/benefit balance changes.

A data-driven review and guide by breed showed that in some large breeds, certain early sterilizations are associated with a higher risk of joint disorders or some cancers, while in other breeds the impact is minimal. This anecdote serves to justify the phrase: “we decide based on breed, size, lifestyle, and actual risks.” Source: Frontiers in Veterinary Science (Hart et al., 2020).

https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2020.00388/full

Anecdotes such as “Did you know?”, “Your vet advises”, “Medical fact”, etc. (additional, non-repetitive information)

Medical fact (dogs): Postoperative “invisible pain” exists… and can be measured.

Many owners believe that “if they don’t cry, it doesn’t hurt.” In veterinary medicine, we use pain scales (assessment of posture, interaction, appetite, response to palpation) and multimodal analgesia. A good practical tip: if your dog is more withdrawn, hunched over, trembling, panting at rest or refusing food, don’t wait, consult your vet: it could be pain or nausea.

(General information based on clinical practice and pain management guidelines; useful as an educational box without repeating what has already been said.)

Your vet advises (dogs and cats): the ’10 days’ are not eternal, but they are decisive.

Repeated licking is one of the most common causes of wound inflammation. Alternatives to the classic Elizabethan collar: soft collar, post-surgical bodysuit, or “donut collar” (depending on species/size). The key is to keep the wound dry, without progressive redness, bad odour, or discharge.

Curiosity (cats): the smell of urine changes after neutering.

Many owners notice less “strong odour” in the litter box. This makes sense: in intact males, urine tends to have more intense compounds linked to sexual marking. It’s a simple, super “everyday” anecdote and very effective for engagement.

Historical fact (dogs): sterilisation as a health tool began long before social media.

Before it became a matter of “opinion,” sterilisation was promoted in cities for public health reasons: control of stray animals, accidents, bites, disease transmission, and shelter overcrowding. It is an interesting approach for a “historical-social” box without getting into moralising.

Practical scientific note (dogs): behaviour ≠ 100% hormone.

If a dog has learned that barking/fighting/escaping “works” for them, neutering may reduce their sex drive, but it does not erase what they have learned. Therefore, when there is aggression or reactivity, the combination that works best is usually: medical examination + sterilisation if appropriate + behavioural modification plan (and, if necessary, an ethologist). This idea adds value without repeating the neutering/vasectomy comparison as it stands.

Sources and recommended reading

Hoffman, J. M., Creevy, K. E., Promislow, D. E. L. (2013) – Reproductive capability, lifespan and causes of death in domestic dogs – PLOS ONE – Large-scale population study showing that neutered dogs live longer and have a different profile of causes of death compared to intact dogs – https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0061082

Torres de la Riva, G., Hart, B. L., Farver, T. B., Oberbauer, A. M., Messam, L. L. McV., Willits, N., Hart, L. A. (2013) – Neutering dogs: effects on joint disorders and cancers in Golden Retrievers – PLOS ONE – Cohort study analysing the relationship between age at neutering, joint disorders and certain cancers in Golden Retrievers – https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0055937

Hayes, H. M. Jr. (1985) – Cryptorchidism and testicular neoplasia in dogs – Journal of the American Veterinary Medical Association – Epidemiological study demonstrating a significant increase in the risk of testicular tumours in dogs with cryptorchidism – https://pubmed.ncbi.nlm.nih.gov/2863879/

Hart, B. L. (1973) – Effects of castration on fighting, roaming, and urine spraying in adult male cats – Journal of the American Veterinary Medical Association – Classic study demonstrating a reduction in sexual and territorial behaviour in male cats following castration – https://avmajournals.avma.org/view/journals/javma/163/3/javma.1973.163.03.290.pdf

Hart, B. L., Hart, L. A., Thigpen, A. P., Willits, N. H. (2020) – Neutering dogs: breed-specific effects on joint disorders and cancers – Frontiers in Veterinary Science – Analysis by breed showing that the risks and benefits of neutering vary according to the size, breed and age of the animal – https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2020.00388/full

Root Kustritz, M. V. (2007) – Determining the optimal age for gonadectomy of dogs and cats – Journal of the American Veterinary Medical Association – Authoritative scientific review of the medical benefits and potential risks associated with neutering according to age – https://avmajournals.avma.org/view/journals/javma/231/11/javma.231.11.1665.xml

Johnston, S. D., Root Kustritz, M. V., Olson, P. N. S. (2001) – Canine and Feline Theriogenology – Saunders – International reference manual on reproduction, endocrinology and sterilisation techniques in dogs and cats – https://www.sciencedirect.com/book/9780721693211