The Truth About Estrogen, Breast Cancer, and Hormone Therapy: What the Latest Research Reveals
No one knows for sure what causes breast cancer to develop, although cancer stem cells (a kind of cell that has nothing to do with estrogen) are thought to play a role. Cancer causes are complex and related to many issues, including inflammation, stress, trauma, exposure to radiation, viruses, chemicals, and other factors.
While genetic mutations have been one of the primary theories driving modern-day Big Pharma cancer treatments, research dating back to 1927 demonstrates that cancer is a metabolic disease. According to many cancer researchers, mitochondrial dysfunction (mitochondria are the energy-producing factories inside our cells) is a hallmark of all cancer cells.
The genetic mutations/abnormalities seen in cancer cells occur secondary to mitochondrial dysfunction. It’s well known that estrogen regulates mitochondrial function and helps repair DNA. That’s probably one of the many reasons pregnancy is breast cancer-protective, since estrogen soars sky-high during pregnancy.
Cancer is “cell growth out of control.” There are multiple regulatory systems inside the body to ensure that cancer cells don’t grow out of control. If a woman develops breast cancer, or any cancer for that matter, it’s a sign that her body’s built-in checks and balances systems aren’t working properly.
If you’re considering HRT, some factors may increase or decrease the risk of breast cancer:
Older women have a higher risk than younger women.
Women who have their first pregnancy before age 20 have a lower risk than women who have their first pregnancy after age 30.
Women who have had multiple pregnancies are at less risk than women with a single pregnancy or those who have never been pregnant.
Breastfeeding decreases risk; the longer a woman breastfeeds, the more protection she gains.
Birth control pill use increases the risk.
Women with greater BMI (body mass index) in post-menopause are at greater risk for developing breast cancer and other cancers. In women who have previously had breast cancer, increasing BMI is also a risk factor for recurrence.
Moderate to heavy alcohol consumption increases the risk for breast cancer (moderate drinking is defined as one or more drinks per day, and heavy drinking is defined as three or more drinks on any day, or more than seven drinks per week).
Women who have issues with detoxification are at greater risk.
Women with a family history (near relatives like mom, aunt, sister) of breast cancer have a small increased risk.
Women with increased breast density have a higher risk than women with less dense breasts.
Women with increased exposure to ionizing radiation (i.e., X-rays, mammograms, UV rays, gamma rays, etc.) have an increased risk for breast cancer.
What does the research show about HRT and breast cancer risk?
Replacing estrogen-alone (including synthetic estrogen, oral estradiol, and transdermal estradiol) does not increase breast cancer risk; it decreases breast cancer risk and mortality.
Estrogen + progesterone does not increase breast cancer risk
Synthetic progestins may increase breast cancer risk
What about women who previously had breast cancer?
Most women have no idea that, for years, estrogen was used to treat metastatic breast cancer. Yep, it’s true. If estrogens were cancer-causing, then this treatment approach would not have worked. Dr. Avrum Bluming and Carol Tavris dedicated the entire first chapter of their book, Estrogen Matters, to answering the question, “Does Estrogen Cause Cancer?” Here’s an excerpt from pages 51-52:
“Surely, the most disconfirming evidence for the claim that estrogen causes breast cancer is this: the administration of estrogen has been shown to have beneficial effects even in women with advanced breast cancer. For example, in 1944 Sir Alexander Haddow, director of the Institute for Cancer Research at the University of London, reported that 25 percent of his patients with advanced breast cancer improved when given high-dose estrogen, and other researchers subsequently have gotten the same or better results. Oncologist Bruno Massidda and his team at the University of Cagliari, Italy, reported remission in 50 percent of advanced breast cancer patients treated with estrogens, and so did Reshma Mahtani and colleagues at the Boca Raton Comprehensive Cancer Center. Gabriel N. Hortobagyi and colleagues at MD Anderson Cancer Center reported that the most effective therapy for metastatic carcinoma of the breast was combined estrogen-progestin. James Ingle and colleagues at Mayo Clinic demonstrated better survival among breast cancer patients treated with diethylstilbestrol (DES), a form of estrogen, compared to tamoxifen, as did Per Eystein Lønning and colleagues at Haukeland University Hospital in Norway. And the pioneer cancer researcher V. Craig Jordan and his research team, demonstrated that both high and low doses of estrogen can shrink cancerous breast tumors. ”
Estrogen, when it’s in proper balance with other key hormones like progesterone and thyroid hormones, does not increase cancer risk. The current data shows that it’s breast cancer-protective. I encourage you to pick up a copy of Estrogen Matters to learn more.
If estrogen does not cause breast cancer, then why are so many women with breast cancer prescribed tamoxifen?
If you know anything about tamoxifen, you know that many people think of it as an estrogen blocker. Tamoxifen is often recommended for its ability to block estrogen receptors in the breasts in women who have estrogen-receptor+ (ER+) breast cancer. What you may not know is that tamoxifen is classified as a human carcinogen and that it raises levels of circulating estrogen in premenopausal women, up to fivefold.
Approximately 40% of women with ER+ breast cancer do not respond to tamoxifen, and many women with estrogen receptor negative (ER-) breast cancer experience a therapeutic benefit from tamoxifen, pointing to tamoxifen’s benefits that have nothing to do with estrogen or its receptors. It’s not fully known how tamoxifen works as a treatment for breast cancer, but it appears that it may inhibit growth factors involved in breast cancer, and work in other ways unrelated to estrogen receptors.
What about HRT in women with ER+ breast cancer?
A common misconception about ER+ cancer is that estrogen caused the cancer. Not so. The term “ER+” stands for estrogen-RECEPTOR positive. ER+ breast cancer tumors are characterized by the presence of estrogen receptors on their cells. Conventionally, this has led to the assumption that estrogen initiates these tumors. If estrogen were the primary driver, breast cancer rates should drop significantly after menopause when estrogen levels plummet, yet the opposite occurs; rates increase with age. This suggests that ER+ tumors are not solely dependent on estrogen for initiation or progression, challenging the idea that they are universally estrogen-driven.
The relationship between estrogen and breast cancer, including ER+ tumors, is far more complex and misunderstood than the mainstream narrative suggests. Estrogen can have protective, neutral, or even therapeutic effects, depending on timing, dosage, and individual factors. Breast cancer initiation and growth are complex and involve a variety of factors, including nonhormonal growth factors that drive tumor growth, progression, and metastasis by influencing cell proliferation, survival, and the tumor microenvironment. And yet, when a woman is diagnosed with breast cancer, the mainstream medical dogma tends to focus solely on estrogen being the “bad guy” without even a mention of all the factors that play a role in cancer tumor growth.
Hormone receptors are found throughout the body. Nearly every organ in our body has receptors for estrogen. That’s how hormones deliver their messages; they “signal” receptors. Our breast cells have hormone receptors for estrogen and progesterone. An ER+ tumor means the breast tumor has estrogen receptors. That’s a good thing because it shows that the tumor cells still have some normal characteristics of a breast cell. These cells usually grow slowly, which is why women with ER+ breast cancer typically have such a good prognosis.
As Dr. Avrum Bluming and Carol Tavris point out in the book, Estrogen Matters, “Indeed, in most breast cancers, estrogen-receptor positive cells are not the ones proliferating.” Dr. Bluming points out in his book that the cells of early breast cancer, as well as the ones that multiply in breast tumors, are typically estrogen- and progesterone-receptor negative. Breast cancer stem cells (BCSCs) are thought to initiate tumor growth and recurrence, and these cells do not have receptors for estrogen or progesterone. These cells do not proliferate in response to estrogen.
We now know that one of the key drivers of recurrence in ER+ breast cancer may not be estrogen at all, but breast cancer stem cells (BCSCs). These are a unique subset of tumor cells that can survive treatment, remain dormant for years, and reignite tumor growth later on. Importantly, BCSCs typically do not have estrogen receptors. This means that they are not fueled by estrogen and do not respond to estrogen-blocking drugs like tamoxifen or aromatase inhibitors.
So, what stimulates breast cancer stem cells?
Emerging research shows that estrogen deprivation itself can enrich these cells. In fact, enrichment of BCSCs can begin within days to weeks of starting estrogen-blocking treatment. Over time, this creates a population of treatment-resistant cells that may later lead to recurrence or metastasis. It takes about a year in lab settings or 4–5 weeks in animal models for endocrine resistance to fully set in, but the groundwork is laid early, during the initial phase of estrogen deprivation. These resistant cells thrive in an environment lacking estrogen. In other words, prolonged estrogen deprivation might actually help the most dangerous cancer cells survive.
This is a crucial point: when a woman is told she must avoid estrogen "forever" after a breast cancer diagnosis, it may actually foster conditions that increase the likelihood of recurrence. This helps explain why some studies have found that carefully reintroducing estrogen (through HRT) may not only be safe but therapeutic in many breast cancer survivors.
In fact, estrogen may help re-establish hormonal balance and immune function, while also limiting the survival advantage of BCSCs. This terrain-focused view moves away from the "kill the cancer cell" approach and focuses instead on rebuilding the body's internal environment in a way that is less hospitable to cancer re-growth.
The takeaway? Blocking estrogen is not a one-size-fits-all solution, and may, in many cases, work against the body’s natural healing and regulatory systems.
What the Science Says About HRT After Breast Cancer
Thus far, there have been 25 published studies of breast cancer survivors given HRT. Four of those studies showed a reduced risk for developing breast cancer when using HRT. None, except the HABITS study (Hormone Replacement After Breast Cancer study), found an increased risk of breast cancer recurrence, and only in women who were taking tamoxifen along with HRT. This implies that ER+ tumors do not automatically "flare up" with estrogen exposure post-diagnosis, countering the fear that estrogen acts as a direct accelerant for these tumors.
The HABITS study consisted of 434 women who were randomized to two groups: a “no-HRT” group and a “treat with HRT” group. It should be noted that the HABITS study was another heavily flawed study that had many issues:
Participants were not required to have any baseline imaging, so no one knows whether the participants already had breast cancer when they began the study.
43 of the 171 women in the placebo (no-HRT) arm of the study decided to use HRT.
11 of the 174 women in the HRT arm of the study decided not to take HRT.
There was no agreed upon HRT regimen for those in the HRT arm of the study — it was left up to the discretion of the provider. (As you now know, the type of HRT matters!).
In the HRT arm, 26 women were found to have breast cancer, compared to 7 women in the no-HRT arm. These cases were limited to local recurrences or contralateral tumors, with no increase in metastatic cancer or death from breast cancer. And again, the HABITS trial, with its many design flaws, was the only one out of 25 studies that showed a slightly increased risk. Due to the study design, small sample size, and other issues, it’s impossible to claim that HRT had anything to do with the increased risk. Yet millions of women have been denied HRT based on this single study when we know that HRT improves quality of life and protects against a multitude of diseases.
You deserve a discussion on the benefits and risks of using (and not using) HRT
Like all treatments, HRT warrants a discussion of risks and benefits. It’s up to you to decide if you feel that the benefits of using HRT outweigh the risks. The decision to use or not use HRT should be a decision based on informed consent, a discussion between you and your provider of benefits and risks based on your unique situation and goals, and what you feel is best for you.
If you’re worried about cancer recurrence (which is understandable), I want you to know that your body has an innate wisdom and ability to heal. Modern medicine often takes a “kill the cancer at all costs” approach without thinking about why cancer developed in the first place. There are reasons cancer develops, and we often need to change the terrain of the body to make it inhospitable to cancer growth and help prevent recurrence.
Killing cancer cells alone is inadequate and short-sighted. Research strongly supports a terrain-based, integrative approach that includes modifying the tumor microenvironment, restoring immune function, reducing inflammation, reducing exposure to toxins (especially hormone-disrupting chemicals), and improving metabolic and psychosocial health. These strategies not only increase treatment effectiveness but also reduce recurrence risk, aligning with a more sustainable vision for cancer care.
What about women who currently have breast cancer?
As you’ve previously read, cancer is complex and caused by multiple factors. Estrogens, when in proper balance with other key hormones like progesterone and thyroid hormones, do not increase cancer risk. In fact, estrogens are critically important for their role in modulating our immune system (i.e., helping our immune system stay in Zen mode).
Currently, even the more conservative medical associations such as The Menopause Society (formerly known as NAMS or North American Menopause Society) recommend that women who have genitourinary symptoms should be offered vaginal estrogen. The American Urological Association recently updated its guidelines on the safety of low-dose vaginal estrogen and DHEA as well. Yet so many women continue to be denied access to any form of HRT.
If you were recently diagnosed with breast cancer, you must build a team of practitioners who are willing to discuss the benefits and risks of your breast cancer treatment, as well as the benefits and risks of hormone therapy. There are risks to not using hormone therapy, and those should be discussed as well.
Women with a diagnosis of breast cancer are often told they shouldn’t consider any HRT until their primary cancer has been treated. But for many women, a treatment regimen may include surgery, chemotherapy, radiation, and tamoxifen/aromatase inhibitors for 10+ years. For a lot of women, these treatments can have devastating physical and mental consequences that significantly impact their relationships and quality of life.
Estrogen is vital to health.
Blocking estrogens systemically (everywhere in the body) will lead to:
Rapid bone loss → increasing risk of fractures and osteoporosis
Cognitive decline → memory, mood, brain fog
Cardiovascular risk → increased risk for heart disease
Genitourinary syndrome of menopause (GSM) → vaginal dryness, painful sex, urinary issues
Metabolic changes → weight gain, insulin resistance
Loss of quality of life → sleep disturbances, hot flashes, fatigue
Women diagnosed with breast cancer while on hormone therapy have consistently been found to have better prognoses than women diagnosed who are not on hormonal therapies. So we need to be thinking about the woman as a whole when discussing the known benefits and risks of HRT.
What about women with a family history of breast cancer?
As I mentioned earlier, there are multiple causes of breast cancer. Genetics only accounts for about 5-10% of breast cancer. Women with a family history of breast cancer should be offered a benefits versus risks discussion with their provider.
What about women who carry the BRCA gene?
Women who carry the BRCA gene have an increased risk of developing breast cancer compared to women who do not carry the BRCA gene. That said, women with the BRCA gene who are on HRT have less risk of developing breast cancer than women with the BRCA gene who do not use HRT. They also have less risk of dying from breast cancer than women who do not use HRT.
I strongly encourage you to pick up a copy of Estrogen Matters, which goes into great depth on the myth surrounding estrogen and breast cancer. Dr. Bluming wrote an excellent summary article published in The Cancer Journal called “Hormone Replacement Therapy After Breast Cancer: It Is Time.” I highly recommend you pick up a copy of this article and print one for your doctor.
Final thoughts
For years, women with a history of breast cancer, especially ER+ breast cancer, have been told to fear estrogen. But emerging science tells a more nuanced story.
Research and real-world clinical experience challenge the outdated belief that estrogen causes or fuels all hormone-sensitive breast cancers. Instead, it explains how hormone receptors function in the body, how tamoxifen actually works (and doesn’t always work), and why the presence of estrogen receptors in tumors isn’t a red flag, but can actually indicate a better prognosis.
Most importantly, it is essential to understand the role of breast cancer stem cells (BCSCs), a treatment-resistant, estrogen receptor-negative cell type that may be the real culprit behind recurrence. Research shows that endocrine therapies like tamoxifen and aromatase inhibitors can enrich these dangerous cells within days to weeks, laying the groundwork for resistance and relapse. Prolonged estrogen deprivation may create a terrain where these cells thrive.
Instead of blanket hormone suppression, it’s time for a personalized, holistic approach that addresses why the cancer started to begin with, and considers how hormone therapy, when used carefully, can support healing, reduce recurrence risk, and improve quality of life after breast cancer.
The takeaway? Estrogen isn’t the enemy. In some cases, it may be part of the solution.
For more information on this topic, see my other articles:
Hormones & Breast Cancer: What’s Fact, What’s Fiction?
The History of Breast Cancer Treatments and the Misguided War on Estrogen
If you’re interested in taking a holistic approach to cancer prevention and treatment, consider finding a provider who specializes in integrative cancer treatment. Some of my favorite resources include: Cancer Doctor, Hope4Cancer, Dr. Leigh Erin Connealy, Dr. Jenn Simmons, and Marnie Clark.
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