Does Hormone Therapy Really Increase Breast Cancer Risk?
The answer depends on which hormone therapy. This is long-term follow-up of two placebo-controlled randomized trials in the Women’s Health Initiative, covering 27,347 postmenopausal women followed for a median of more than 20 years. Prior randomized use of estrogen alone, in women who had already had a hysterectomy, was associated with 22% lower breast cancer incidence and 40% lower breast cancer mortality than placebo. Prior randomized use of estrogen plus a progestin, in women with an intact uterus, was associated with 28% higher breast cancer incidence than placebo, and breast cancer deaths did not differ significantly between the two groups.
Dr. Kumar’s Take
This study is the reason I refuse to discuss hormone therapy as a single yes-or-no question. Two randomized trials, run in the same program, in the same era, produced opposite signals depending on whether a progestin was part of the regimen. Estrogen alone did not raise breast cancer risk in these women, it lowered it, and it lowered breast cancer death. The combination raised incidence, and the annual rates on both sides of that comparison were still under half a percent. That is the number a woman deciding about treatment for disabling menopausal symptoms deserves to hear, not a headline about a 28% increase with no denominator attached.
Study Snapshot
This analysis followed 27,347 postmenopausal women aged 50 through 79, all with no prior breast cancer and a negative baseline screening mammogram, enrolled at 40 US centers between 1993 and 1998. In the trial of 16,608 women with a uterus, 8,506 received 0.625 mg/d of conjugated equine estrogen plus 2.5 mg/d of medroxyprogesterone acetate and 8,102 received placebo; that trial stopped in 2002 after a median 5.6 years of treatment. In the trial of 10,739 women with a prior hysterectomy, 5,310 received 0.625 mg/d of conjugated equine estrogen alone and 5,429 received placebo; that trial stopped in 2004 after a median 7.2 years of treatment. Follow-up ran through December 31, 2017, more than 20 years of median cumulative follow-up, with mortality information available for more than 98% of participants. The primary outcome was breast cancer incidence, with deaths from breast cancer and deaths after breast cancer as secondary outcomes.
Results in Real Numbers
- Estrogen alone, incidence: 22% lower risk of breast cancer, 238 cases at an annual rate of 0.30% versus 296 cases at 0.37% on placebo (p = .005)
- Estrogen alone, breast cancer death: 40% lower, 30 deaths at an annual rate of 0.031% versus 46 deaths at 0.046% (p = .04)
- Estrogen plus progestin, incidence: 28% higher risk, 584 cases at an annual rate of 0.45% versus 447 cases at 0.36% on placebo (p < .001)
- Estrogen plus progestin, breast cancer death: 71 deaths at an annual rate of 0.045% versus 53 deaths at 0.035%, not a statistically significant difference (p = .11)
- Scale of the absolute rates: every one of these annual event rates, in every group, was below half of one percent per year
Who Benefits Most
Women who have had a hysterectomy and can take estrogen alone had the most favorable results here: lower breast cancer incidence and lower breast cancer mortality than placebo. For women with an intact uterus, who need a progestin alongside estrogen, incidence was higher than placebo, though the annual rates remained low and breast cancer deaths did not differ significantly. That split is the practical basis for individualizing the decision rather than applying one rule to every woman.
Safety, Limits, and Caveats
These results describe two specific regimens at two specific doses: conjugated equine estrogen at 0.625 mg/d, and that same estrogen combined with medroxyprogesterone acetate at 2.5 mg/d. Participants had a mean baseline age of 63.4 years, older than most women beginning treatment for perimenopausal symptoms, and all entered with a negative screening mammogram and no history of breast cancer. Treatment itself lasted a median of 5.6 years in the combination trial and 7.2 years in the estrogen-only trial, with the rest of the two decades being observation after the trials stopped. Women with a strong family history of breast cancer or other elevated risk still need individual assessment.
Practical Takeaways
- Ask which regimen is being discussed before you decide anything, because estrogen alone and estrogen plus a progestin pointed in opposite directions in these trials
- If you have had a hysterectomy, estrogen alone was associated with less breast cancer and fewer breast cancer deaths than placebo
- If you have an intact uterus, the combination was associated with 28% higher incidence, from an annual rate of 0.36% to 0.45%
- Read percentage increases against the underlying annual rate, which stayed below half a percent in every group here
- Recognize that these findings apply to the two formulations and doses actually tested, at a mean starting age of 63.4
Related Studies and Research
Risks and benefits of hormone therapy: has medical dogma now been overturned?
Episode 31: Depression Explained, The Biology Behind the Darkness
Episode 32: Depression Recovery Roadmap: A Step-by-Step, Evidence-Based Plan
Perimenopausal depression: review of recent findings and implications for future research
Episode 27: Perimenopause, Menopause, and HRT - What Every Woman Should Know
FAQs
Should I avoid hormone therapy because of breast cancer risk?
Not automatically. In these trials, estrogen alone in women with a prior hysterectomy was associated with 22% lower breast cancer incidence and 40% lower breast cancer mortality than placebo. The combination was associated with 28% higher incidence, from an annual rate of 0.36% to 0.45%, without a statistically significant difference in breast cancer deaths. Those are different conversations.
How does this apply to other hormone formulations?
This study tested conjugated equine estrogen at 0.625 mg/d, alone or with medroxyprogesterone acetate at 2.5 mg/d. The results describe those regimens. Bring your specific prescription to a clinician who treats menopause regularly.
What if I have a family history of breast cancer?
Everyone in these trials entered with no prior breast cancer and a negative screening mammogram, so the results describe that starting point. A family history calls for individual assessment of your risk alongside your symptoms.
Bottom Line
Twenty years of follow-up on two randomized trials produced two different answers. Estrogen alone, in women with a prior hysterectomy, was associated with less breast cancer and fewer breast cancer deaths than placebo. Estrogen plus a progestin, in women with an intact uterus, was associated with 28% higher incidence, an annual rate of 0.45% versus 0.36%, and breast cancer deaths in that comparison did not differ significantly. That supports an individualized decision built around which regimen a woman actually needs.
