What if I told you that you could reduce your breast cancer risk by a whopping 39%? It might sound too good to be true, but this massive reduction was achieved through hormone replacement therapy in a major study involving 1268 patients.

In this post, I'll walk you through the fascinating findings of the Dayton Study—a prospective observational study that explored the effects of testosterone therapy on breast cancer risk in women. Over a decade of observation, the study uncovered some unexpected results that could change the way we think about hormone replacement therapy. We'll delve into the details of the study, discuss the results and side effects, and consider what these findings mean for women considering testosterone therapy.

Stay with me as we explore how testosterone could be a game-changer in reducing breast cancer risk and what you need to know to make informed decisions about your hormone health.

1. Overview of the Dayton Study

When discussing groundbreaking research in hormone replacement therapy (HRT), the Dayton Study stands out for its forward-thinking approach and its long-term observation of breast cancer outcomes in women undergoing testosterone therapy. This study is notable because it focuses exclusively on the effects of testosterone, without the inclusion of other hormones like estrogen, which are commonly part of HRT protocols.

The Dayton Study was designed as a prospective observational study, a type of research that involves following participants over a period of time to observe outcomes without altering their treatment regimens. In this case, the study tracked 1268 women over a span of 10 years, making it one of the more comprehensive studies in the field of hormone replacement.

The primary objective of the Dayton Study was to evaluate the impact of testosterone therapy on breast cancer risk. The participants, all women in menopause, received testosterone pellets implanted under the skin. These pellets gradually release testosterone, maintaining a steady hormone level. Notably, the study did not include any estrogen or other hormone replacement interventions, allowing researchers to isolate the effects of testosterone.

One of the unique aspects of the Dayton Study is that it did not have a placebo group. Instead, the outcomes of the women receiving testosterone were compared to age-matched controls from the SEER (Surveillance, Epidemiology, and End Results) database. This comparison group, managed by the National Cancer Institute, provided a robust dataset for understanding cancer incidence in the general population.

In addition to focusing on breast cancer outcomes, the study also monitored various side effects of testosterone therapy, providing a comprehensive view of its impact on menopausal women. This approach ensured that the benefits and potential drawbacks of testosterone therapy were both thoroughly examined.

By the end of the study, the data revealed some compelling results that have significant implications for women considering testosterone therapy as part of their HRT regimen. These findings, which we'll dig into next, highlight the potential of testosterone to reduce breast cancer risk, alongside other notable effects.

2. Study Results and Side Effects

The Dayton Study provided some eye-opening results regarding the impact of testosterone therapy on breast cancer risk and its various side effects. These findings are crucial for understanding the potential benefits and drawbacks of incorporating testosterone into hormone replacement therapy for menopausal women.

Reduction in Breast Cancer Risk

One of the most significant outcomes of the Dayton Study was the remarkable reduction in breast cancer risk among the participants. The study found that women who received testosterone therapy had a 39% lower incidence of breast cancer compared to age-matched controls from the SEER database. Specifically, the SEER database indicated an incidence rate of 271 cases of breast cancer per 100,000 women, while the women in the study had an incidence rate of only 165 cases per 100,000. This substantial reduction underscores the potential protective effect of testosterone against breast cancer, a finding that could have profound implications for women considering hormone replacement therapy.

Common Side Effects

While the reduction in breast cancer risk is a promising finding, the study also highlighted several side effects associated with testosterone therapy. Understanding these side effects is essential for women and healthcare providers to make informed decisions about hormone therapy.

Increased Facial Hair

A notable 85% of women in the study experienced increased facial hair. This side effect, while common, varied in severity:

  • Mild Cases: Most women had mild increases in facial hair that were manageable with typical hair removal methods.
  • Severe Cases: About 6% of the participants reported severe episodes of increased facial hair, which required more intensive management.


Acne was another common side effect, reported by approximately 11% of the women. This side effect is consistent with the known effects of androgens (male hormones) on the skin, leading to increased oil production and potential acne outbreaks.

Voice Changes

Voice changes were reported by around 1% of the participants. These changes included:

  • Cracking Voice: Similar to the voice changes experienced during puberty, some women had episodes of their voice cracking.
  • Deepening of Voice: In rare cases, women experienced a deepening of their voice, which could be distressing, especially for those who sing or speak professionally. My wife, a professional worship leader, has experienced significant vocal changes with testosterone that required dose adjustments.

Skin Improvements

Interestingly, about 50% of the women reported improvements in their skin, including:

  • Reduced Wrinkles: Some participants noticed a decrease in the appearance of wrinkles.
  • Improved Skin Moisture and Texture: Many women reported their skin felt more hydrated and had a better overall texture.

3. Testosterone Levels and Comparisons

The Dayton Study highlighted the high initial levels of testosterone achieved through the use of implanted pellets. This section will delve into these levels and their implications, as well as comparisons made with age-matched controls from the SEER database.

High Initial Testosterone Levels

Testosterone levels in women normally fluctuate but are significantly lower than those typically observed in men. For women of childbearing age, normal testosterone levels are around 100 ng/ml. However, in the Dayton Study, participants experienced much higher levels due to the testosterone pellets. These pellets were designed to maintain a steady release of testosterone over several months.

  • Initial Levels: Four weeks after implantation, the average testosterone level in the participants was about 300 ng/ml.
  • Sustained Levels: Even four months after implantation, testosterone levels remained elevated at approximately 171 ng/ml.

These elevated levels are much higher than the typical physiological range for women, which might explain some of the more pronounced side effects observed in the study.

Comparison with Age-Matched Controls

One of the unique aspects of the Dayton Study was its use of age-matched controls from the SEER (Surveillance, Epidemiology, and End Results) database. This comparison allowed researchers to contextualize the results within a broader population framework.

  • SEER Database: Managed by the National Cancer Institute, the SEER database is a comprehensive source of cancer incidence and survival data in the United States. It provided a robust dataset for comparison.
  • Control Group: The study compared the incidence of breast cancer in women undergoing testosterone therapy with those in the SEER database who did not receive such treatment. This comparison was crucial in highlighting the potential benefits of testosterone therapy.

The use of age-matched controls helped underscore the significant reduction in breast cancer risk observed in the study participants. However, it is important to note that the comparison also introduced some potential biases, which will be discussed in the next section.

4. Breast Cancer Risk Reduction Explained

The Dayton Study's most compelling finding was the 39% reduction in breast cancer risk among women receiving testosterone therapy. This section will explain the data in detail and its implications.

Detailed Explanation of the Risk Reduction

The study's primary goal was to assess the impact of testosterone therapy on breast cancer incidence. The results were remarkable:

  • SEER Data: The SEER database indicated an incidence rate of 271 cases of breast cancer per 100,000 women in the general population.
  • Study Data: Among the women in the Dayton Study, the incidence rate was significantly lower at 165 cases per 100,000 women.

This 39% reduction in breast cancer risk is a substantial finding, suggesting that testosterone therapy may offer protective benefits against breast cancer. Such a reduction has the potential to change the way we approach hormone replacement therapy for menopausal women.

Implications of These Findings

The significant reduction in breast cancer risk observed in the Dayton Study suggests that testosterone therapy could be a viable option for reducing the incidence of breast cancer in menopausal women. However, it is essential to consider these findings within the broader context of hormone therapy and individual health profiles.

  • Potential Benefits: Testosterone therapy could be particularly beneficial for women at higher risk of breast cancer or those seeking alternatives to traditional hormone replacement therapies that include estrogen.
  • Clinical Considerations: Healthcare providers should weigh the potential benefits of testosterone therapy against the observed side effects and individual patient needs. Personalized treatment plans are crucial for optimizing outcomes.

The findings from the Dayton Study provide a strong foundation for further research into the benefits of testosterone therapy and its role in reducing breast cancer risk. However, as with any study, it is important to consider the limitations and potential biases that might affect the results.

5. Criticisms and Caveats

While the Dayton Study offers promising insights into the benefits of testosterone therapy, it is not without its criticisms and caveats. This section will address some of the key concerns and limitations of the study.

Researchers' Overly-Cautious Approach to Estrogen

One notable aspect of the Dayton Study was the researchers' overly-cautious approach to estrogen. The study excluded estrogen from the treatment regimen due to concerns about its potential to stimulate breast tissue and increase breast cancer risk. This decision was influenced by historical data and prevailing medical opinions, particularly the misinterpretation of findings from the Women's Health Initiative (WHI).

  • Estrogen and Breast Cancer: The WHI initially appeared to suggest an increased risk of breast cancer with estrogen-progestin therapy, leading to widespread caution in hormone therapy practices. However, more recent and thorough analyses have challenged these findings, suggesting that the risks may have been wildly overstated.

Use of Aromatase Inhibitors

To mitigate any potential conversion of testosterone to estrogen, the study included the use of anastrozole, an aromatase inhibitor, in participants with a history of breast cancer. Aromatase inhibitors block the production of estrogen from testosterone, further reducing the risk of estrogen-related breast tissue stimulation.

  • Impact on Results: The use of aromatase inhibitors could have influenced the study outcomes by eliminating any estrogenic effects, thereby isolating the impact of testosterone.

Potential Healthy User Bias

Another significant caveat of the Dayton Study is the potential for healthy user bias. Participants who opted for testosterone therapy might have been more health-conscious and proactive in managing their health compared to the general population.

  • Health Behaviors: These women may have had better diets, exercised more regularly, and avoided smoking, all of which could contribute to a lower breast cancer risk independent of the testosterone therapy.
  • Comparison Limitations: Comparing study participants to the general population in the SEER database may not fully account for these differences in health behaviors and lifestyle, potentially skewing the results.

Limitations of the Study Design

As an observational study, the Dayton Study has inherent limitations that must be considered when interpreting the results.

  • Lack of Randomization: The absence of a randomized control group means that the study cannot definitively establish causation, only associations.
  • Single Treatment Arm: Without a placebo or alternative treatment group, it is challenging to isolate the effects of testosterone alone.

6. Practical Considerations and Recommendations

The findings of the Dayton Study present compelling evidence that testosterone therapy can significantly reduce breast cancer risk in menopausal women. However, these benefits must be weighed against the potential side effects and the unique health profiles of individual patients. Here are some practical considerations and recommendations for those considering testosterone therapy.

Safety and Potential Benefits

The potential for a 39% reduction in breast cancer risk is a significant benefit of testosterone therapy. For many women, this could be a compelling reason to explore this treatment option, particularly if they are at higher risk for breast cancer or are seeking alternatives to traditional hormone replacement therapies.

  • Breast Cancer Prevention: The dramatic reduction in breast cancer incidence observed in the Dayton Study suggests that testosterone could play a protective role. Women with a family history of breast cancer or other risk factors might find this particularly beneficial.
  • Symptom Relief: Beyond cancer prevention, testosterone therapy can also alleviate common menopause symptoms such as low libido, fatigue, and mood swings. Improved skin texture and reduced wrinkles are additional benefits that some women may find appealing.
  • Improvement in Bone Density: Testosterone therapy can also contribute to improved bone density, which is crucial for preventing osteoporosis, a common concern for menopausal women. Stronger bones mean a lower risk of fractures and better overall health.

Consulting a Hormone Optimization Specialist

Given the complexities of hormone therapy, it is crucial to consult with a hormone optimization specialist. These professionals are trained to evaluate individual health profiles and provide personalized treatment plans that maximize benefits and minimize risks.

  • Expert Guidance: A hormone specialist can help you understand your hormone levels, recommend the appropriate type and dosage of testosterone, and monitor your progress. They can also address any side effects you might experience and adjust your treatment accordingly.
  • Personalized Treatment Plans: Hormone optimization is not a one-size-fits-all approach. A specialist will tailor your therapy to your specific needs, considering factors such as your overall health, hormone levels, and personal preferences.

How to Find a Qualified Specialist

Finding a qualified hormone optimization specialist can be challenging, but there are resources available to help.

  • Referrals: Your primary care provider or gynecologist might be able to recommend a specialist. Additionally, professional organizations and online directories can be valuable resources.
  • Simple Hormones Referral Program: By filling out a simple form on my website, I may be able to help connect you with experienced hormone optimization practitioners in your area. While I can't guarantee a specialist in every town, I'll do my best to find someone who can meet your needs.

Monitoring and Adjusting Therapy

Regular monitoring is essential to ensure the effectiveness and safety of testosterone therapy.

  • Regular Check-ups: Regular follow-ups with your healthcare provider are crucial. These visits allow your provider to monitor your hormone levels, assess the effectiveness of the therapy, and make necessary adjustments.
  • Addressing Side Effects: If you experience side effects such as increased facial hair, acne, or voice changes, your specialist can adjust your dosage or recommend strategies to manage these issues.

Testosterone - The Bottom Line

The Dayton Study provides promising evidence that testosterone therapy can significantly reduce breast cancer risk while also offering other benefits for menopausal women. However, it's essential to consider the potential side effects and individual health profiles when deciding on this treatment.

If you're ready to take the next step, it's essential to find a hormone optimization specialist, someone with the experience, training, and expertise to know exactly what to do with all your hormones, including testosterone. Fill out my referral form and request a referral to a hormone specialist. Take charge of your health and well-being today by partnering with a professional who can guide you through the complexities of hormone therapy and help you achieve optimal health.

Take control of your hormone health today and explore the potential benefits of testosterone therapy. By working with a knowledgeable specialist and adopting healthy lifestyle habits, you can navigate menopause with greater confidence and well-being.

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About the Author

With over 26 years of experience as a licensed pharmacist in Utah and Colorado, I specialize in hormone optimization and menopause management. I hold certifications in Advanced Bioidentical Hormone Replacement Therapy (ABHRT) from Worldlink Medical, C4 Hormone Replacement Therapy from the Professional Compounding Centers of America and the American Academy of Anti-Aging Medicine, and I am a Brain Health Coach certified by Amen Clinics.

I also share my expertise on my Simple Hormones YouTube channel where my videos have been viewed over 950,000 times and over 18,000 viewers have subscribed.

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