Abstract
Neutering consists of removing the source of the hormones that control reproduction and determine secondary sexual characteristics. In dogs and cats, this is most commonly accomplished by castration or ovariectomy. While the primary purpose of neutering is to prevent reproduction, the procedure may have other physical and behavioural effects. Epidemiologic research has identified many beneficial and harmful outcomes associated with neutering. A definitively causal relationship between these outcomes and neuter status cannot be accepted without consistent evidence from multiple studies of adequate size and quality. However, consideration of the possible health consequences of these associations is warranted when offering owners recommendations concerning neutering. An evidence-based decision about neutering a particular pet requires integrating relevant research data with the veterinarian's clinical expertise and the needs and circumstances of the patient and owner. It is impossible to precisely predict the outcome of neutering for any individual. However, existing research does allow some generalization about the magnitude and clinical importance of specific risks and benefits. Overall, it appears justified to recommend spaying all females not intended for breeding, because the procedure is more likely to prevent rather than cause disease. In male dogs, the benefits of castration are not so clearly greater than the risks. The evidence is also mixed regarding the risks and benefits of neutering dogs before 5-6 months of age, and so no strong recommendation for or against the practice can be made. However, it is clear that spaying female dogs before their first heat is preferable to spaying them later.