Study: Estrogen without progesterone for hormone replacement may lower breast cancer risk
Women with average breast cancer risk who took estrogen without progesterone after menopause had lower rates of breast cancer than those who did not take any hormone replacement. This finding comes from studies looking at the safety of the use of estrogen alone as hormone replacement therapy. (Posted 2/13/24)
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RELEVANCE
Most relevant for: Postmenopausal people on hormone replacement therapy.
It may also be relevant for:
- healthy people with average cancer risk
Relevance: Medium-High
Strength of Science: Medium-High
Research Timeline: Post Approval
What is this study about?
This study combines data from one large and nine small studies to determine how estrogen-only hormone replacement therapy after menopause affects the risk of developing breast cancer.
Why is this study important?
Prior research has shown that plus progesterone hormone therapy after menopause increases the risk of breast cancer. However, while some studies have shown that taking estrogen without progesterone increased the risk of breast cancer, others reported that it decreased breast cancer risk.
This new study looks at the results of multiple past studies; it included only high-quality research known as controlled trials. These types of trials help researchers compare multiple groups of people who received different types of medical care to determine whether their outcomes differed.
The large Women’s Health Initiative (WHI) study compared the outcomes of women who took estrogen without progesterone to those who did not did not take any hormone replacement. Participants who took estrogen alone were less likely to develop breast cancer and less likely to die of breast cancer than those who did not take any hormone replacement. In contrast, two other large studies found that estrogen alone increased a woman’s risk of developing breast cancer.
Study findings
Women who took estrogen-only hormone therapy after menopause have lower rates of breast cancer than those who did not take hormone therapy. These results come from an analysis of 10 randomized control trials. The largest—the Women’s Health Initiative—had the most (10,739) participants. Together, the nine smaller studies included 3,500 participants.
Researchers found that:
- In the combined 10 studies, women who took estrogen were 33% less likely to develop breast cancer than those who did not take hormone therapy.
- At 10-year follow-up; results from the WHI study showed that estrogen alone compared to no hormone therapy decreased breast cancer risk by 37%.
- Results for the 9 smaller studies were similar to the early results from the Women’s Health Initiative data. Together, the results from these studies showed that participants who took estrogen alone had a 35% lower risk of developing breast cancer.
- Women who started estrogen earlier saw a stronger protective effect, according to the data from the WHI.
- The type of estrogen taken (conjugated equine estrogen, Estradiol or estrone sulfate) did not affect the study results.
At the time of enrollment, all participants in the Women’s Health Initiative study:
- were postmenopausal
- had a hysterectomy
- had never been previously diagnosed with breast cancer
- had a normal
The other studies had similar criteria. However, some studies included women with a history of breast cancer, although no conclusions were made about this group.
Participants took estrogen for varying amounts of time, ranging from an average of two years in some studies to over seven years in the WHI study. WHI participants were followed for more than 20 years.
