Breast Cancer Risk Associated with Hormone Therapy

Synthetic Progestin–Containing Combined Hormone Therapy

Combined estrogen + synthetic progestin therapy increases breast cancer risk (relative risk ≈ 1.4; 95 % CI 0.7–1.2) and adds roughly 8 invasive cancers per 10,000 women‑years 1, 2.
In the Women’s Health Initiative, estrogen + medroxyprogesterone acetate raised breast cancer incidence (hazard ratio ≈ 1.26; 95 % CI 1.00–1.59) and produced a higher proportion of node‑positive, advanced‑stage tumors 1, 2.
Continuing synthetic‑progestin regimens beyond five years further elevates risk (relative risk ≈ 1.23–1.35) 1.

A personal history of breast cancer is an absolute contraindication to any systemic hormone‑replacement therapy, irrespective of hormone‑receptor status or progestogen type 3.
Systemic HRT must not be prescribed to women with a prior breast‑cancer diagnosis, even when micronized progesterone is used 3.

For high‑risk women without a personal breast‑cancer history, micronized progesterone should be the preferred progestogen when combined HRT is deemed necessary, owing to its more favorable breast‑cancer safety profile 3.
First‑line progestogen dosing: micronized progesterone 200 mg orally at bedtime, either continuously or for 12–14 days each cycle 3.

Estrogen‑alone therapy does not increase breast cancer risk and may be modestly protective (hazard ratio ≈ 0.80; 95 % CI 0.62–1.04) 2.
This underscores that the excess breast‑cancer risk observed with combined HRT is driven primarily by the progestogen component, especially synthetic agents 2.

The Women's Health Initiative (WHI) study found a 26% increased incidence of breast cancer in women using combined estrogen-progestin therapy, with a hazard ratio of 1.26 (95% CI, 1.00-1.59) 4
Breast cancers were more advanced in the treatment group compared to the placebo group, although the increased risk declined rapidly after cessation of HRT 4
The Heart and Estrogen/Progestin Replacement Study (HERS) observed similar increases in breast cancer incidence with an RH of 1.27 (95% CI, 0.84-1.94) over 6.8 years of follow-up 5, 6
The WHI study of women who had undergone hysterectomies and were receiving unopposed estrogen did not show a significant increase in breast cancer risk 4, 7
Long-term follow-up of the WHI estrogen-only arm showed no significant difference in breast cancer incidence between treatment and placebo groups (HR, 0.80; 95% CI, 0.62-1.04; P = .09) 4
The Million Women Study showed an association between current use of estrogen-only HRT and increased risk of breast cancer (relative risk, 1.30; 95% CI, 1.21-1.40; P < .0001) 7, 8
The Nurses' Health Study demonstrated significantly increased breast cancer risk after long-term use (≥20 years) of estrogen alone (relative risk, 1.42; 95% CI, 1.13-1.77) 7, 8

The National Comprehensive Cancer Network (NCCN) recommends against the use of HRT for women taking tamoxifen or raloxifene outside of a clinical trial 8
The U.S. Preventive Services Task Force (USPSTF) concluded there is fair to good evidence that HRT increases the incidence of breast cancer, particularly with estrogen plus progestin 5, 9