Study Finds Testosterone May Help Neutered Male Dogs

The study, published in BMC Veterinary Research in July 2025, states that a critical gap in veterinary medicine: the lack of safety and dosing information for restoring hormone levels in neutered dogs to treat a range of health and behavioural issues collectively termed “spay-neuter syndrome“. Surgical gonadectomy (spaying or neutering) removes the reproductive organs, leading to a loss of hormones like estradiol and testosterone, which can cause many serious health conditions and behavioural changes, including increased prevalence of cancer, urinary incontinence, obesity, hypothyroidism, and increased aggression, fearfulness, and anxiety. While in alternative, hormone-sparing sterilisation methods are emerging, little is known about treating dogs already neutered.

The primary purpose of this study was to provide basic safety and dosing information for injectable testosterone cypionate [a drug commonly used to increase testosterone levels in males with hypogonadism, a condition where the body doesn’t produce enough testosterone] in neutered male dogs over a three-month period. The researchers aimed to evaluate any changes in health and behaviour, and whether testosterone supplementation altered endocrine measures.

Twelve previously neutered male dogs (six Beagles and six Foxhounds), averaging 2.9 years of age and castrated at an average of 7.7 months, participated in the study. Researchers divided the dogs into four equal groups and administered weekly subcutaneous injections of testosterone cypionate at 0 (control), 1x, 3x, or 5x the standard target dose of 0.5 mg/kg for 90 days. They collected a range of data, including bloodwork (endocrine assays, hematology, and chemistries), body condition scores, and prostate health using the Zambelli Symptom Index. The kennel staff, who conducted daily behavioral assessments, remained blinded to the treatment groups to prevent bias.

Key Safety and Efficacy Findings:

The study showed that giving dogs weekly testosterone injections for three months was pretty safe overall. Side effects were uncommon and mostly mild – we’re talking about things like a swollen toe, some skin irritation, and two seizures in one dog. The dog that had seizures already had a history of suspected idiopathic epilepsy (basically seizures with no known cause), which is actually pretty common in Beagles due to genetics. So those seizures were probably not related to the testosterone treatment at all. Even when they gave dogs five times the normal dose, no other problems came up.

The testosterone levels went up in a predictable way based on the dose given. Dogs that got 3x and 5x the standard dose actually managed to get their testosterone back into the normal range you’d see in intact dogs (0.5-9.0 ng/ml). Even the group getting the 1x dose showed some improvement, though it wasn’t statistically significant compared to the control group. They measured levels one week after each shot, so the levels might have been even higher mid-week.

When dogs are neutered, they often have really high LH (luteinizing hormone) levels because there’s no negative feedback from the hormones their testicles would normally produce. In this study, the 5x dose group showed a significant drop in LH levels starting around day 28 compared to baseline, and the 3x group also had lower levels by day 90 (though not statistically significant). This proves that LH does respond to testosterone supplementation, but getting back to normal levels for neutered dogs only happened at the highest dose. So they might need additional treatment, like a GnRH agonist (a hormone that affects reproduction), to fully bring LH levels down if that’s what you want.

Even though there were concerns about testosterone potentially causing benign prostatic hyperplasia or BPH (enlarged prostate), the study didn’t find any BPH symptoms when they used the Zambelli Score to assess it. This could be because of how long the treatment lasted, the age of the dogs, or how they measured things, but it suggests BPH might not be as big a problem with hormone restoration as people thought. Still, they recommend keeping an eye on prostate health regularly.

There wasn’t much variation over time or between groups when it came to body condition scores, clinical evaluations (heart rate, temperature, body weight), and routine blood work. No significant changes showed up in estradiol, cortisol, thyroid stimulating hormone, or thyroid hormone levels. There was also no evidence of secondary erythrocytosis (polycythemia – basically too many red blood cells), which is a common side effect in humans on testosterone therapy. Hemoglobin and hematocrit levels stayed within normal ranges.

While aggressive interactions were pretty rare overall, five of the nine recorded incidents happened in the 5x dose group. But because there were so few incidents, small numbers of subjects, and uncontrolled factors, the study couldn’t really draw solid conclusions about whether testosterone therapy actually affects canine behavior. They’d need a more rigorous behavioral study to figure that out.

In conclusion, this initial target safety study provides valuable foundational data for veterinarians considering testosterone restoration in neutered male dogs. It demonstrates that weekly subcutaneous injections of testosterone cypionate are safe and effective at increasing testosterone levels to within the normal range for intact dogs over a three-month period, even at higher doses. While high doses can reduce elevated LH levels, additional treatments like GnRH agonists may still be needed for complete normalisation. The study’s limitations include small group sizes and no long-term follow-up, highlighting the need for further research to clarify long-term outcomes and account for factors like breed, age, and size.