Read Dr. Sherry Weaver's advice.
I would start by saying that there are many different vets with as many different experiences and opinions. In these articles, I try to express my personal experiences as well as current research. No research is perfect, and no experience is absolute. In reality, there are very few definite "right" answers. The best answer to most questions is to find a vet who you trust to tell you the whole story and make your decision based on that information.
Early neutering is a controversial topic. A very few uncontrolled studies have shown a link with early neuters (before 14 months of age) and some forms of cancer and joint problems. Both the joint problems and the cancers that they have linked are relatively common in large-boned dogs, so the challenge is to prove whether the early neuter actually caused an increase in the incidence. There have been no studies that prove this.
On the other hand, there have been several good studies done to look at different potential complications from early neutering (as early as 6 weeks of age) that have found no adverse effects other than slightly longer legs and less "masculine" muscle development. These pro-early neuter studies were not carried out long enough to evaluate the risk of cancer.
In my experience, dogs neutered under 6 months develop less obesity and don’t establish some of the “male” behaviors that neutering is meant to treat. They are also less likely to jump a fence, fight, or get hit by a car. With no true proof of the risk of increased cancer--and the very real risk of bad behaviors or even physical injury from running away or fighting--I am letting my clients make their own decisions based on a debatable risk of cancer vs. the very real behavioral risks.
Spaying females before 6 months is less controversial than neutering; preventing the first heat nearly eliminates the risk of breast cancer which is much more common than bone cancer.
Dr. Sherry Weaver
Dr. Weaver graduated with honors from the University of Georgia's School of Veterinary Medicine. She founded a state-of-the-art animal hospital, teaches pet care to children, and donates time and resources to rescue organizations.