AAHA recommends that all dogs not intended for deliberate breeding be spayed or castrated. For pet owners who choose to breed, practitioners should promote responsible breeding practices including collaboration with research programs to reduce perpetuation of disease through careful selection of breeding individuals.

Sterilized dogs of both sexes have greater average lifespans than intact dogs.^95–100 Sterilization also changes the trajectory of disease acquisition and causes of death.^95,100–103 Better data are needed about the optimal age to sterilize dogs of particular sizes and breeds in order to optimize benefits to health and longevity while minimizing risk of certain diseases.

Although there is emerging data from multiple studies, there is insufficient evidence to draw firm conclusions regarding the relationship between orthopedic disease and the timing of sterilization.^104–109

Because sterilized dogs live longer, and neoplasia is more frequent at later ages, studies on the effects of sterilization on the diagnosis of neoplasia must be carefully constructed to avoid the confounding risk of age.⁹⁷

Risk of certain cancers may be increased or decreased by sterilization timing, varying by tumor and breed.

• Sterilized dogs are at higher risk than intact dogs for anal gland carcinoma, prostate cancer, osteosarcoma, hemangiosarcoma, transitional cell carcinoma, and mast cell tumors.^110–115
• One study showed that the later female rottweilers were spayed, the lower their risk of osteosarcoma.¹¹¹
• Early-sterilized male golden retrievers had greater risk of lymphoma than late-neutered or intact males, with no sterilization effect on risk among females, while sterilized vizslas of both sexes had greater risk than intact vizslas.^109,116
• Conversely, late-sterilized female golden retrievers and early-sterilized vizslas had greater risk of hemangiosarcoma, with no sterilization effect on risk among male golden retrievers and a latesterilization risk for male vizslas.^109,116
• Sterilization was associated with increased risk of mast cell tumor in golden retriever females, with no effect in males, but in vizslas of both sexes.^109,116
• Risk of mammary cancer in female dogs shows a reverse effect of timing; spaying prior to first estrus minimizes risk, and risk increases with the number of estrus cycles up to 2.5 yr of age.^117,118,119
• Most, but not all, studies in dogs identify a risk for obesity among sterilized individuals, and the risk of development of obesity in sterilized dogs was greatest in the 2 yr following surgical sterilization.^120–123
• Urethral sphincter mechanism incompetence (USMI) is more common overall among spayed females.^124–126
• Some, but not all, studies suggest that USMI develops more commonly among females spayed prior to first estrus than those spayed later.^{102,122,127,128}

A summary of the recommended timing for canine spaying and neutering is summarized here. 

• Female and male dogs not intended for breeding and expected to be <45 lbs when full-grown should generally be sterilized by 5–6 mo of age. This age range can be further individualized to the patient with the goals of spaying prior to the first estrus in females to decrease the risk of mammary neoplasia and correcting persistent deciduous teeth during one anesthetic event.
• As a result of the possible orthopedic concerns, certain cancers in some breeds, and the phenotypical differences between large and giant breeds, males expected to be >45 lbs should be sterilized when growth is complete, usually between 9 to 15 mo.There may be a benefit to orthopedic health of postponing even longer.
• The recommendations are less clear in female dogs expected to be >45 lbs; therefore, clinical discretion and extensive client communication (concepts to consider) and education will be needed to individualize the care in this population. These recommendations attempt to balance risk of orthopedic disease, USMI, and some cancers associated with early sterilization, against risk of mammary neoplasia, unwanted litters, and possible other cancers if sterilized later (balancing risks). These medical recommendations may need to be balanced against certain nonmedical extenuating circumstances, such as likelihood of future access to veterinary care, financial incentives provided by adoption groups, or the opportunity to perform surgical sterilization concurrently with another anesthetized procedure.