Hormones & Cancer

Rethinking Breast Cancer Risk: The Truth About Hormones and Prevention

By Julie Taguchi, MD, Hematologist-Oncologist

For years, women have been told that hormone therapy increases the risk of breast cancer—a long held belief since the discovery of the estrogen receptor was discovered on breast cancer cells and more recently, largely rooted in the Women’s Health Initiative (WHI) study from the early 2000s. Unfortunately, this narrative is incomplete. The truth is the word “hormones” can have several meanings so different hormone regimens have different outcomes.

In my work as an oncologist, I’ve spent decades researching and treating breast cancer, and I believe it’s time to take a more nuanced, evidence-based approach to the role of hormones in breast health. My personal journey, including the co-authoring of Sex, Lies, and Menopause, and my clinical practice in Santa Barbara which included an observational study (unpublished), has continually reinforced this need.

Let’s break down what the objections to the WHI, the real risks for breast cancer, and why hormonal rhythms may actually help protect against breast cancer.

What the WHI was not About Hormones and Breast Cancer

The Women’s Health Initiative (WHI) was one of the most influential studies on hormone therapy, but its findings were initially misinterpreted and miscommunicated to both doctors and the public.

WHI was not designed to study hormone therapy for menopause—it was primarily a cardiovascular study.
The WHI did not include perimenopause women. The average age of women in the WHI was 63—meaning they been in menopause, often over a decade prior.
The WHI used Premarin (CEE) and medroxyprogesterone acetate (MPA) as the “hormone” intervention—not bioidentical estradiol or progesterone.
The WHI reported the infamous 26% increase in breast cancer risk from Prempro (CEE + MPA) actually translated to just 8 more cases per 10,000 women per year. Those women had the cancer in progress before the study started. No increased deaths.
The WHI Premarin-only group (CEE alone) actually had a 23% reduction in breast cancer risk after 6 years of use and this benefit held for 20 years.

In other words:

Estrogen alone did NOT increase breast cancer risk—it actually lowered it.
Progestins (synthetic progesterone-like drugs) and or the continuous administration may have been the issue, not estrogen itself.

Despite this, the WHI headlines terrified women and doctors, leading to a massive decline in hormone therapy use—even for women who could have benefited from it. Studies now suggest that between 2002 and 2014, more than 150,000 women may have died prematurely due to hormone avoidance.

Understanding Breast Cancer Risk: The Bigger Picture

Breast cancer is complex and involves multiple factors beyond hormones.

The Real Risks for Breast Cancer

The biggest and common risk factor for breast cancer is AGE. If you’re a woman and you’re having birthdays, you’re accumulating risk.

Other key risk factors include:

Dense breast tissue – One of the strongest predictors.
Family history of early-onset breast cancer.
BRCA1 and BRCA2 gene mutations.
High-dose radiation exposure.
First full-term pregnancy after age 40 – A significant but under discussed risk.

One of the biggest risk factors for breast cancer is having your first baby after age 40. That’s not politically correct to say, but it’s the truth.

Hormones and Breast Cancer: What the Research Shows

Not all hormones are created equal.

Synthetic progestins like MPA have been linked to increased breast cancer risk.
Continuous combined regimens show more risk than cyclic ones.
Bioidentical estradiol and progesterone do not appear to increase risk and may, in fact, have protective effects when administered correctly.
Estradiol and progesterone, when given in physiologic rhythm, activate tumor-suppressor genes and reduce inflammation in research studies.

Estradiol wants to protect the woman. Estrogen keeps women healthy so they can reproduce.

A Patient’s Journey with Hormone Therapy After Breast Cancer

In 2007, I treated a 67-year-old woman diagnosed with grade three estrogen receptor-positive DCIS. She couldn’t tolerate anti-estrogen therapy and joined my study using rhythmic hormone replacement. For years, she thrived. Even after a second diagnosis and invasive HER2 positive cancer at 74, she chose to resume hormone therapy to maintain her quality of life.

She is now 84, thriving and recurrence-free.

Her story mirrors many others—patients who understand the risks, make informed decisions, and benefit tremendously from properly administered hormones.

Most women who had early breast cancer were saying, ‘I would rather have those robust active years with hormones than without.’

The Protective Role of Hormone Rhythms

Emerging science and clinical experience suggest that hormonal rhythm matters.

Peak levels of estradiol and progesterone stimulate P53, the “guardian” tumor suppressor.
Estradiol reduces oxidative stress and inflammation.
Progesterone promotes apoptosis in abnormal breast cells.
Mimicking natural menstrual rhythms may preserve these protective effects.

When hormone levels are high—especially estradiol and progesterone—they regulate and help prevent cancer. That’s their job. There is more cancer when women do not have hormones.

Hormone Therapy After Cancer: A My Nuanced View

Women with triple-negative breast cancer (ER-, PR-, HER2-) may consider the use of hormone therapy post-treatment.
Postmenopausal women on hormone therapy at the time of diagnosis should not be rushed to discontinue until after surgery or unless it is contraindicated due to therapy.
Hormone receptor-positive breast cancer requires more nuanced consideration, including Oncotype DX scores and, ideally, a Breast Cancer Index to assess estrogen sensitivity.
Ductal Carcinoma in Situ (DCIS) that has been completely treated can be good candidates
A second cancer is always possible since there has been one already.

A completely treated tumor in an older woman with strong ER positivity often has a favorable prognosis.

Beyond Breast: Brain, Bone, and Longevity

Hormones aren’t just about hot flashes.

Estradiol supports brain health, bone density, and cardiovascular function.
In one case, full reversal of osteoporosis—from a T-score of -3.0 at age 57 to normal—was observed over 10 years of rhythmic hormone therapy.
Research shows increased estrogen receptor density in brain regions as women approach menopause, indicating the brain’s need for estrogen.

Hormones equal a better life. They’re not the enemy—they’re a tool for vitality when used wisely.

Final Thoughts: A New Era of Women’s Health

For decades, women have been denied hormone therapy out of fear. It’s time to reclaim the narrative with clarity, compassion, and science.

The WHI created unnecessary fear.
Not all “hormones” are hormones or are equal.
Rhythmic hormone therapy may offer protection, not just relief.
Women deserve more than outdated rules—they deserve choices backed by evidence.

Let’s move forward—together—with truth, trust, and transformation.