Does neutering always cause weight gain?

No, but it makes weight gain very likely if nothing changes. After surgery maintenance energy requirement drops and in many animals appetite increases, so the previous ration becomes excessive. Reducing the amount from the day of surgery and weighing the animal monthly keeps the weight stable without any particular difficulty.

Is it better to neuter before or after the first season?

It depends on species and size. In queens and small breed bitches there is no documented advantage in waiting, and protection against mammary tumours is greater the earlier surgery happens. In large and giant breed bitches the choice often moves to after growth is complete for orthopaedic reasons, but that requires managing seasons carefully and knowing the signs of pyometra.

Does neutering fix behaviour problems?

Only those driven by sex hormones: roaming, urine marking, mounting and part of the conflict between intact males. Fear based aggression, separation anxiety and lead reactivity are not solved by surgery and can in some cases get worse. Those problems need a behavioural assessment, before and independently of the operation.

Are there alternatives to surgery?

In males there are slow release implants that temporarily suspend hormone production and are also used as a reversible trial, to see which behaviours would actually change before committing. There are also techniques that keep the gonads while preventing reproduction, less widespread and with specific indications. These are options to discuss with your vet, and they are not equivalent to each other.

My male dog is healthy: should I castrate him anyway?

No, there is no general clinical obligation. In males the main benefits concern prevention of non tumoural prostatic disease, removal of testicular tumour risk and reduction of some sexual behaviours. If none of those areas is a problem, the decision is a management one and should consider breed, size and living context.

Health and prevention9 min readUpdated on July 30, 2026

Neutering: what really changes in long term risk

Pyometra, mammary tumours, incontinence, weight and orthopaedic risk: what the evidence says about neutering and about when to do it.

Audience: Pet owners
Species: Dog, Cat
Scope: Valid everywhere, European Union, Italy

Neutering is almost always presented as a binary decision taken once, while in practice it is a decision with two variables: whether to do it and when. Over the past fifteen years the veterinary literature has dismantled the idea that the answer is the same for every animal, because measured effects differ sharply between dogs and cats, between males and females, between small and large breeds, and in some cases between one breed and another. Some benefits are solid and barely disputed, some risks are real and rarely mentioned, and several widely quoted studies have methodological limits worth knowing about. This article separates what is documented, what is a statistical association, and what remains a management choice.

What is removed and what changes in the body

In females the ovaries are removed, on their own or together with the uterus. The two techniques are equivalent for preventing pyometra, because cyclic hormonal stimulation is what causes it and without ovaries that stimulation no longer exists. In males the testicles are removed. In both cases the effect is not only sterility: sex hormone production stops, and beyond reproduction those hormones influence metabolism, growth plate closure, urethral sphincter tone and some aspects of behaviour.

This is where both the benefits and the biological costs of surgery come from, and it is why the same operation has a different balance depending on the age at which it is done. An animal neutered before puberty does not go through the hormonal changes that close growth plates on schedule, and grows with slightly longer long bones. In a cat or in a four kilogram dog this produces no measurable consequence; in a forty kilogram dog the resulting joint geometry has been associated with a different orthopaedic risk.

The documented benefits

The most solid benefit in females is the elimination of pyometra, the uterine infection that typically appears in the weeks after a season and that is a surgical emergency. Scandinavian insurance cohorts, which follow large populations for years, report that roughly one intact bitch in four develops pyometra by ten years of age, with strong differences between breeds. Removing the ovaries takes that risk down to essentially zero.

Pyometra: risk removed in the female, and the benefit grows with age because incidence rises over the years.
Mammary tumours: in bitches about half of mammary neoplasms are malignant, in queens the malignant share is far higher and clinical behaviour is aggressive.
Testicular tumours: risk removed in the male, with the largest benefit in cryptorchid animals, where a retained testicle has a markedly higher chance of transformation.
Benign prostatic hyperplasia and other non tumoural prostatic disease: greatly reduced incidence in the castrated male.
Unwanted litters, repeated false pregnancies, escapes and conflicts linked to seasons or to females in season nearby.

The link between early neutering and reduced mammary tumour risk needs one methodological caveat. The classic figure, according to which risk rises with every season before surgery, comes from studies that are widely quoted and very old; a systematic review published in 2012 judged that evidence weak because of risk of bias, without contradicting the direction of the effect. Translated for someone who has to decide: the protection very probably exists, its exact size is less certain than it is usually presented.

The risks that get less airtime

None of these risks is an automatic reason to skip surgery, but knowing they exist allows you to prevent them or catch them early rather than discover them as surprises.

Unwanted effects reported after neutering and how they are handled
Effect	Who it affects most	What can be done
Weight gain	Every animal, dog and cat, in the months right after surgery	Cut the ration from the day of surgery and weigh monthly
Urinary incontinence from sphincter incompetence	Medium and large breed females, sometimes years later	Recognise leaking during sleep and treat it, response to therapy is good
Cruciate ligament rupture and hip dysplasia	Large breed dogs neutered before growth is complete	Choose timing by breed and size rather than one universal rule
Coat changes	Long coated breeds with a heavy undercoat	A cosmetic effect with no clinical consequence

On the oncological side there are reported associations between gonadectomy and certain neoplasms, for example osteosarcoma, haemangiosarcoma and some lymphoid forms, but the data depend heavily on breed and in several cases come from hospital populations rather than the general population. Breed specific analyses show a mosaic: in some breeds orthopaedic risk rises with early neutering, in others no difference appears, in others still it differs between males and females. This is why the correct answer to when to neuter is itself a question: which breed, which size and which sex are we talking about.

Timing matters as much as the decision

Until a few years ago the standard was to operate before the first season in every female and at around six months in every male. Today the choice is adjusted above all for size, because the time needed to complete skeletal growth ranges from a few months to well over a year and a half.

What weighs on the timing decision, by group of animals
Group	What matters most	Common approach
Cats, male and female	Unwanted litters, roaming, fighting, disease spread through bites	Early surgery, the balance is clearly favourable
Small breed dog	Pyometra and mammary tumours in the female, growth completed early	No documented advantage in a long delay
Large breed dog, female	Long growth period, orthopaedic risk, incontinence	Individual assessment, often after growth is complete
Large breed dog, male	Long growth period, orthopaedic risk, behaviour management	Individual assessment, clinical urgency is almost always low

In large breed dogs choosing to wait carries a real management commitment: in females it means dealing with one or more seasons, controlling outings for three weeks and watching for signs of pyometra over the following two months. Delaying without organising for it is not an informed decision, it is a postponement.

In cats the picture is simpler

For cats the controversies that apply to large dogs do not apply. Growth completes early, data linking early neutering to orthopaedic problems do not exist in the same form, and the risks of life as an intact cat are immediate and documented: roaming far from home, road accidents, fight wounds and viral disease transmitted through bites, urine marking indoors in males, repeated litters in females with any outdoor access.

A queen can come into season repeatedly for weeks without mating, with continuous calling and considerable stress.
Feline mammary tumours are malignant in the large majority of cases, and neutering at a young age reduces the risk.
An intact male marking indoors is not a training problem: it is sexual behaviour, and it decreases after castration in most animals.
In cats living in groups neutering reduces conflict and therefore infected wounds and abscesses.

Decide, then monitor

On behaviour it is important to hold correct expectations. Castration reduces behaviours driven by sex hormones, meaning roaming, marking, mounting and part of the conflict between intact males. It does not change behaviours that arise from fear, and some observational studies have actually found greater noise reactivity and fearfulness in animals neutered early. A dog that growls because strangers frighten him does not improve with surgery: he improves with a behaviour plan.

Bring your animal's data to the conversation
Breed, expected adult size, sex, lifestyle, known conditions and the behaviours that worry you. These are what shift the timing decision, not a general rule.
Ask which technique and why
In females, removal of the ovaries alone or of the uterus as well, open or laparoscopic approach. Ask also how pain will be managed in the following forty eight hours, which is the part that most affects recovery.
Cut the ration from day one
Agree the new amount before surgery and apply it straight away. Waiting to see the weight climb means having to remove far more later to get back.
Weigh monthly for the first year
A monthly weight together with a body condition assessment catches drift while it is still a few hundred grams. Record them together, because the number alone says little.
Note what to watch for later
In medium and large breed females, damp patches where she sleeps are the typical sign of sphincter mechanism incompetence, which responds well to treatment. It is not a training problem and it should not be tolerated.

Frequently asked questions

Does neutering always cause weight gain?: No, but it makes weight gain very likely if nothing changes. After surgery maintenance energy requirement drops and in many animals appetite increases, so the previous ration becomes excessive. Reducing the amount from the day of surgery and weighing the animal monthly keeps the weight stable without any particular difficulty.
Is it better to neuter before or after the first season?: It depends on species and size. In queens and small breed bitches there is no documented advantage in waiting, and protection against mammary tumours is greater the earlier surgery happens. In large and giant breed bitches the choice often moves to after growth is complete for orthopaedic reasons, but that requires managing seasons carefully and knowing the signs of pyometra.
Does neutering fix behaviour problems?: Only those driven by sex hormones: roaming, urine marking, mounting and part of the conflict between intact males. Fear based aggression, separation anxiety and lead reactivity are not solved by surgery and can in some cases get worse. Those problems need a behavioural assessment, before and independently of the operation.
Are there alternatives to surgery?: In males there are slow release implants that temporarily suspend hormone production and are also used as a reversible trial, to see which behaviours would actually change before committing. There are also techniques that keep the gonads while preventing reproduction, less widespread and with specific indications. These are options to discuss with your vet, and they are not equivalent to each other.
My male dog is healthy: should I castrate him anyway?: No, there is no general clinical obligation. In males the main benefits concern prevention of non tumoural prostatic disease, removal of testicular tumour risk and reduction of some sexual behaviours. If none of those areas is a problem, the decision is a management one and should consider breed, size and living context.

What to do next

Take three pieces of information to your next appointment: expected adult size, breed and the behaviours that worry you, and ask your vet for the recommended timing for that specific profile rather than a general rule. If surgery has already happened, agree the new ration immediately and set a monthly weigh in for the first year, because post neutering weight gain builds quietly during the first six months.