Flawed Studies Linked Menopausal Hormone Therapy to Breast Cancer | The Scientist

Although menopause is not a new phenomenon by any means, the past few years of what some have called the “menopause moment” have brought it to the forefront of national conversation

One form of treatment, called menopausal hormone therapy, can address several bothersome symptoms, particularly hot flashes, and can improve quality of life for some patients. It can also provide an added layer of heart health and bone protection for some

Despite these benefits, prescriptions steeply declined after the Women’s Health Initiative – the largest and longest-running study on women’s health in the U.S. since the early 1990s – found an increased risk of breast cancer among those on menopausal hormone therapy. This created substantial fear around using hormone therapy among providers and patients alike

However, inconsistencies and biases in the study’s findings have arisen over the years, leading researchers to scrutinize the data more closely. Furthermore, the Food and Drug Administration began removing the black box warnings – which signal serious health risks – on hormone therapy for menopause in November 2025

What does this mean for the 1-in-8 women who will be diagnosed with breast cancer in their lifetime?

The FDA’s change in safety warnings highlights a gray zone on who would benefit from menopausal hormone therapy and which formulations carry the lowest risk. As a gynecologist who specializes in breast cancer care, I believe these new guidelines emphasize the need to tailor treatment to each individual – including breast cancer survivors for whom menopausal hormone therapy has historically been avoided.¹

Menopause and Breast Cancer Survivors

Breast cancer is the most commonly diagnosed cancer among women in the United States. Breast cancer diagnoses are increasing among women under 50, with a 1.4 percent increase each year between 2012 and 2021

Breast cancer survivors may suffer from symptoms of naturally occurring menopause due to their chronological age. However, they may also be experiencing menopausal symptoms as a result of breast cancer treatments. For younger survivors, breast cancer treatments could induce menopause by temporarily or permanently disrupting ovarian function.² Sometimes the ovaries may be removed for those at increased risk for ovarian cancer.

These effects can impair or disable the body’s ability to produce hormones like estrogen, which increases a young breast cancer survivor’s risk of cardiovascular disease and osteoporosis.^3,4

Menopausal hormone therapy could mitigate the health risks that young breast cancer survivors face.5 Estrogen-containing therapies are linked to not only decreased cardiovascular and all-cause mortality among younger patients, but also to improved quality of life. They are also the most effective treatment for hot flashes, one of the most bothersome symptoms of menopause

Breast cancer survivors represent a significant portion of the population who could benefit from menopausal hormone therapy. However, healthcare providers often avoid this treatment due a perception that it would increase a breast cancer patient’s risk of recurrence

But the research is conflicting

Deconstructing the Data

Some of the fear surrounding menopausal hormone therapy was shaped by findings from the Women’s Health Initiative study, which researchers have since critiqued for its limitations

In particular, while the study did find that people taking menopausal hormone therapy had an elevated risk of breast cancer, this was among a group of older participants between ages 50 to 79. This led to the timing hypothesis, which proposes that some groups may experience more benefit and less risk from menopausal hormone therapy based on their age.⁶

Results also differed by treatment formulation.7 The most commonly used version of progesterone used today is safer than the version used in the Women’s Health Imitative.8 Breast cancer risk increased among those using estrogen plus progesterone, but was not increased among those taking estrogen alone

The Women’s Health Initiative also did not specifically look at how menopausal hormone therapy affected cancer risk in breast cancer survivors

Deconstructing Even More Data

Another landmark study called “Hormone Replacement Therapy After Breast Cancer – Is It Safe?,” or HABITS, evaluated the safety of menopausal hormone therapy for those previously diagnosed with breast cancer.⁹ The study ended early in 2003, after 26 out of 219 women in the group receiving menopausal hormone therapy developed breast cancer, in comparison with the 8 of 215 women who did not receive hormone therapy.

Much like the Women’s Health Initiative, the HABITS trial – along with a similar study called the Stockholm Trial – gained much attention by suggesting that menopausal hormone therapy was unacceptably risky for breast cancer survivors.¹⁰

However, researchers have likewise criticized the HABITS trial for possible discrepancies in the treatment formulations given to patients.11 Additionally, the breast cancer characteristics of some participants were unknown, which may have affected the data

A 10-year follow-up of the trial ultimately revealed no significant difference in breast cancer recurrence between survivors who did or did not receive menopausal hormone therapy.10 Both the HABITS and Stockholm trials also did not show an increased death rate related to menopausal hormone therapy among breast cancer survivors

The findings suggest that the increased recurrences seen in the HABITS trial may have been due to higher progesterone exposure. Ultimately, because the studies were terminated early, no definitive conclusions can be made

Conflicting Research

Numerous other studies have produced variable results

A smaller clinical trial did not show significantly increased breast cancer risk in survivors with estrogen-negative tumors who were receiving only estrogen over a five-year period.¹²

A study pooling data from four clinical trials showed a significantly increased risk of recurrence for survivors with hormone receptor-positive tumors who were receiving estrogen and progesterone, but did not see an increased risk for those with hormone-negative tumors.¹³

There are also more than 20 studies published over the past several decades that found no significant increased risk of new breast cancer activity, recurrences or death for survivors taking menopausal hormone therapy.11 Some of these studies even suggest a decreased risk of breast cancer with certain formulations of menopausal hormone therapy

But these studies have not received the same amount of attention and consideration. The Women’s Health Initiative and the HABITS trial were both larger randomized trials, which are gold standards in research. However, the limitations of these trials and the conflicting data of other studies create challenges in how to interpret these findings

Personalized Care for Breast Cancer Survivors

Removing the black box warning on menopausal hormone therapy does not mean it is safe or appropriate for every patient at any age. Rather, it encourages a conversation between the patient and healthcare provider about their eligibility and individual risks

Research has suggested that different risk factors may play a role in breast cancer risk, although it is unclear how each factor ultimately interacts with one another. Factors such as age, breast cancer subtype and hormone therapy formulation could affect someone’s overall risk in different ways.¹⁴

Given this complexity, a risk-benefit analysis and shared decision-making on treatment could offer a more practical and holistic approach to if, when and under what conditions breast cancer survivors could be eligible for menopausal hormone therapy

Precision medicine, which personalizes medicine and tailors care to the individual, has become the mainstay of healthcare. I believe the removal of the black box warning on menopausal hormone therapy represents an important opportunity to continue improving holistic, personalized care for breast cancer survivors

Versha Pleasant, Clinical Assistant Professor; Director, Cancer Genetics & Breast Health, University of Michigan

This article is republished from The Conversation under a Creative Commons license. Read the original article

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