Estrus Suppression in Dogs: Choosing the Right Medical Option While Minimizing Risks

Unlike surgical sterilization, medical estrus suppression offers a reversible alternative for bitches intended for future breeding¹. The ideal treatment would effectively prevent estrus, preserve fertility, and have minimal adverse effects. Unfortunately, no currently available medication fulfills all three criteria. 

The choice of therapy therefore requires balancing efficacy against potential short- and long-term consequences. 

Progestogens: The Traditional Approach 

Progestogens have been used for canine estrus suppression for more than 70 years and remain among the most commonly used medical options. 

Commonly used agents include²: 

• Medroxyprogesterone acetate (MPA)

• Megestrol acetate (MA)

• Chlormadinone acetate

• Delmadinone acetate

• Proligestone

These drugs are typically administered during anestrus, approximately one month before the expected onset of the next heat cycle. 

How Effective Are They? 

Megestrol acetate has demonstrated high efficacy, suppressing estrus in approximately 92% of bitches when administered during early proestrus. Proligestone can suppress estrous cycles for extended periods, with the first post-treatment estrus often occurring 9–12 months later and occasionally taking several years to return². 

The Downside 

While effective, progestogens are associated with the greatest number of adverse effects among available options. 

Commonly reported effects include: 

• Increased appetite and weight gain

• Lethargy

• Restlessness

More concerning complications include: 

• Insulin resistance and diabetes mellitus

• Growth hormone excess and acromegaly-like changes

• Skin and coat abnormalities

• Cystic endometrial hyperplasia

• Endometritis and pyometra

Long-term administration may also increase the risk of mammary gland pathology. Progestogen-induced mammary changes can progress from hyperplasia to adenomatous lesions and, in some cases, malignant transformation. 

For this reason, veterinarians should carefully assess the reproductive value of the bitch and discuss risks thoroughly before initiating prolonged treatment. 

Androgens: Effective but Less Popular 

Synthetic androgens have also been used to suppress estrus. 

The best-known example is mibolerone, which can postpone estrus for extended periods when treatment begins at least 30 days before proestrus. In some cases, suppression may last up to two years^1,3. 

Once treatment is discontinued, most bitches return to estrus within one to seven months. 

What Limits Their Use? 

The primary concern is androgen-related side effects. 

These include: 

• Clitoral enlargement

• Vaginitis

• Increased body odor

• Urine marking

• Mounting behavior

• Thickening of the skin

Additionally, mibolerone should not be used in Bedlington Terriers because of an increased risk of liver disease. It is also contraindicated in dogs with androgen-responsive tumors². 

Although effective, these adverse effects have limited widespread acceptance of androgen-based suppression protocols. 

GnRH Agonists: A Modern Alternative 

The introduction of long-acting gonadotropin-releasing hormone (GnRH) agonists has significantly expanded options for reversible fertility control. 

Products containing deslorelin acetate have gained considerable attention because they can provide prolonged estrus suppression while maintaining future fertility². 

Understanding the Flare-Up Effect 

One unique feature of GnRH agonists is the initial "flare-up" response. 

Instead of immediately suppressing reproduction, these drugs initially stimulate release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), resulting in: 

• Vulvar swelling

• Vaginal bleeding

• Behavioral signs of heat

• Ovulation

After this brief stimulation phase, the pituitary becomes desensitized, leading to prolonged suppression of ovarian activity. 

When administered during anestrus, deslorelin implants may suppress estrous cycles for up to 27 months. 

Can the Flare-Up Be Prevented? 

Several studies have investigated combining megestrol acetate with deslorelin implants to reduce or eliminate the initial induced estrus. Results suggest that carefully timed administration of megestrol acetate can significantly reduce this unwanted effect. 

Potential Complications 

Reported adverse effects include: 

• Persistent estrus

• Ovarian cyst formation

• Induced lactation

• Behavioral changes

• Occasional pyometra

Although uncommon, these risks highlight the importance of monitoring patients after implantation. 

GnRH Antagonists: The Emerging Option 

Unlike GnRH agonists, GnRH antagonists immediately block pituitary GnRH receptors. 

This provides one major advantage: 

No flare-up effect. 

Because gonadotropin release is suppressed immediately, estrous signs do not develop following treatment. 

One such compound, acyline, has demonstrated the ability to suppress estrus for approximately three weeks when administered during early proestrus⁴. 

Why Aren't They Used More Often? 

The main limitation is practicality. 

Most GnRH antagonists require repeated injections and relatively high doses, making them less convenient than long-acting implants. Nevertheless, they remain an area of active research and may play a larger role in future canine reproductive management. 

Which Option Is Best? 

There is no universally ideal drug for estrus suppression in dogs². 

• Progestogens are highly effective but carry significant risks involving the uterus, mammary gland, and metabolism.

• Androgens provide reliable suppression but are limited by masculinizing side effects.

• GnRH agonists offer long-term reversible suppression and preservation of fertility but are associated with an initial flare-up response.

• GnRH antagonists avoid flare-up effects but currently lack convenient long-acting formulations.

The best choice depends on the age, breed, intended breeding future, medical history, and owner expectations for each individual patient. 

Key Takeaway 

Medical estrus suppression can be a valuable reproductive management tool, but it should never be considered a one-size-fits-all solution. Understanding the benefits and limitations of each pharmacological option allows veterinarians to tailor treatment plans while minimizing risks to future fertility and overall health. As newer hormonal therapies continue to evolve, the focus remains on achieving effective, reversible estrus control with fewer adverse effects than traditional approaches. 

References

1. McRae G, Runcan E, Johnson A. Estrous cycle manipulation in dogs. Clinical Theriogenology. 2025 Feb 14;17:82-6. https://clinicaltheriogenology.net/index.php/CT/article/download/11647/18904

2. Kutzler MA. Estrus suppression in dogs. Veterinary Clinics: Small Animal Practice. 2018 Jul 1;48(4):595-603. https://doi.org/10.1016/j.cvsm.2018.04.001

3. Roos-Pichenot J, Zakošek Pipan M. “My Bitch Is Empty!” an Overview of the Preconceptional Causes of Infertility in Dogs. Veterinary sciences. 2025 Jul 12;12(7):663. https://www.mdpi.com/2306-7381/12/7/663

4. Lindh L, Luvoni GC, Peltoniemi O, Lindeberg H, Goericke-Pesch S. The oestrous cycle and inhibition of oestrus in canids. https://helda.helsinki.fi/server/api/core/bitstreams/e3a65f44-4642-412f-8072-1d9f9e7bfc79/content