When to desex?

Cheryl being speyed

Historically, cats and dogs were always desexed (castrated or speyed) at about 6 months of age. This was to allow the patient to develop as much as possible physically but to try and desex them before they had their first heat. The timing of the first heat varies with species and breed, and it is not infrequent that cats speyed at 6 months of age are already pregnant.

This traditional timing for desexing has been reviewed over the last few years, as all long-standing policies should be, and subjected to close examination as to whether the date is best for the patient. We tend to desex from 4 months of age.

Much has been written about purported risks of desexing, especially of desexing young. Let’s look at some of the evidence.

Why do we desex?

  • To prevent unwanted litters
  • To reduce roaming and fighting behaviour, and therefore injury and spread of infectious disease
  • Speying dogs and cats can reduce the risk of mammary cancer. See note below.
  • Speying dogs prevents pyometra. This is a hormonally-driven infection of the uterus, which affects around 25% of female dogs during their lifetime. Untreated, pyometra is fatal.
  • Castration of dogs dramatically reduces the chances of benign enlargement of the prostate gland, which affects most entire male dogs by late middle-age, and causes problems with urinating and defaecating.
  • Castration of dogs reduces the chance of perineal herniation, and dramatically reduces the chance of tumours around the anus. It also completely prevents cancer of the testicles.
  • Desexed cats and dogs tend to live longer than entire ones.

What are the risks of desexing early?

  • It takes 12 weeks on average for a cat or dog to develop sufficiently to be able to withstand an anaesthetic as safely as an adult, in terms of maintaining blood pressure and temperature while asleep, and being able to remove the anaesthetic drugs from their bodies.
  • Some cancers have shown a mild increase in risk with early desexing in large breed dogs. Examples are osteosarcoma (bone cancer), haemangiosarcoma (blood vessel cancer), prostatic carcinoma (prostate cancer).
  • Risk of damage to the cruciate ligaments in the knee is mildly increased.

And the benefits?

  • Speying young dramatically reduces the risk of mammary (breast) cancer. Unspeyed bitches have about 26% chance of developing mammary cancer, of which about half is malignant. If they are speyed after having only had one heat, the risk drops to 8%, and if they are speyed before any heats, the risk drops to 0.5%.
  • Speying female cats before 6 months of age reduces the risk of mammary cancer 7 times, and speying at any age still reduces the risk about 50%.
  • Speying is technically easier when the dog or cat is younger.
Chloe being speyed

So, what are our recommendations?


There is much less time pressure on male dogs to be desexed young. It may prevent or reduce roaming behaviour, and in that sense is best done before this behaviour starts or very soon thereafter. We recommend castration from 4 months of age, but it can be delayed longer if required, especially if roaming behaviour is not yet an issue.

Female dogs are a completely different story due to the enormous benefit of early desexing on mammary cancer risk. Statistically, this completely outweighs the small increase in the risk of other conditions and represents a very significant health benefit. We recommend desexing before the first heat, so normally around 4-5 months of age.

  • Female dogs spey prior to 5 months of age.
  • For large-breed male dogs house pets – orthopaedic concerns may outweigh all others – spay/neuter after growth stops 15–18 months.
  • For large-breed male dogs free-roaming – population concerns may outweigh all others – spay/neuter prior to 5 months of age.
  • For small-breed male dogs – no evidence at this time for orthopaedic issues – castrate prior to sexual maturity – 5 months.


Female cats frequently come in heat before 6 months of age, which results in stray tomcats arriving at your house. We recommend speying at 4-5 months of age.

Male cats left entire present a huge problem in terms not only of producing unwanted pregnancies but especially with the fighting behaviour that spreads diseases such as Feline Immunodeficiency Virus (FIV). We recommend male cats are castrated at 4-5 months of age.

Paul Eason BVM&S MANZCVS (Surgery; Emergency and Critical Care Medicine)


Franklin Vets

Franklin Vets - excellence in veterinary care for dairy, farming, lifestyle, equine and household pets. BESTPRACTICE ACCREDITED NZ.