In the healthcare and scientific communities, systematic reviews are utilized to guide medical decisions as they provide an exhaustive summary of high-quality peer-reviewed studies relevant to a research question. The foundation for our segments which outline the positive and negative effects of spay/neuter is a 2007 systematic review of more than 50 peer reviewed studies ⁸³ in the veterinary medical literature detailing the long term physical risks and benefits associated with spay/neuter in dogs. This review concludes:

“On balance, it appears that no compelling case can be made for neutering most male dogs, especially immature male dogs, in order to prevent future health problems. The number of health problems associated with neutering may exceed the associated health benefits in most cases.”

On the positive side, neutering male dogs:

• eliminates the small risk (probably <1%) of dying from testicular cancer
• reduces the risk of non-cancerous prostate disorders ⁴⁷
• reduces the risk of perianal fistulas ⁴⁶
• may possibly reduce the risk of diabetes (data inconclusive)

On the negative side, neutering male dogs:

• if done before 1 year of age, significantly increases the risk of osteosarcoma (bone cancer); this is a common cancer in medium/large and larger breeds with a poor prognosis. ^{13,14,60,61,109}
• increases the risk of cardiac hemangiosarcoma (an aggressive cancer of the blood vessels lining the heart) by a factor of 1.6 ^{4,25,60,61,109}
• triples the risk of lymphosarcoma in golden retrievers when neutered before 12 months ^60,61,86,109
• increases the risk of lymphoma and lymphosarcoma¹⁹⁰
• triples the risk of hypothyroidism ^26,27,28,29
• increases the risk of progressive geriatric cognitive impairment ⁵⁵
• triples the risk of obesity, a common health problem in dogs with many associated health problems ^30,31,32
• quadruples the small risk (<0.6%) of prostate cancer ^5,6,7,88,109
• doubles the small risk (<1%) of urinary tract cancers ²³
• increases the risk of orthopedic disorders ^{30,42,48,49,50,51,52,53,60,61,64,67,68,89,108}
• increases the risk of adverse reactions to vaccinations ³⁴
• triples the risk of atopic/allergic dermatitis ⁹²
• triples the risk of patellar luxation ⁵⁴

One of the objectives of the veterinary experts when they initiated the practice of neuter was to eliminate the testosterone in male dogs as it was seen as a hormone that made dogs aggressive, and caused male dogs to exhibit negative behavior such as humping, and wandering. However, it is no exaggeration to say that in modern human medicine and endocrinology testosterone is no longer a marginal hormone. Neither is it a lifestyle hormone for those men seeking eternal youth. Its deficiency leads to a serious deterioration of the health of men expressing itself in the metabolic syndrome and its sequels: diabetes mellitus type 2 and atherosclerotic disease, osteoporosis and sarcopenia, all strongly limiting physical independence in old age and accelerating morbidity and mortality.¹²⁸

Likewise, according to the Textbook of Veterinary Internal Medicine, intact dogs who develop an enlarged prostate gland have been treated with hormonal analogues (gestagens) which effectively “chemically castrate” these dogs. The desired clinical outcome of reversing the enlargement of the prostate gland is predictably achieved. However, the side effects can be severe and include diabetes mellitus, hypothyroidism, adrenal insufficiency, and mammary gland hyperplasia. Again we ask, why would veterinarians continue to treat testosterone as an unnecessary and/or problematic hormone and continue to recommend neutering (physical castration)?

As a footnote to this discussion, we want to point out that conventional neuter surgeries are invasive procedures, and involve removal of the testes.  A Canadian study concluded that the total complications (intraoperative and postoperative) for neuter surgeries were  about 19%.⁴  Vasectomy, as we recommend, could be expected to have fewer complications as it is a much less invasive procedure and does not remove the testes.