- All types of menopausal hormone replacement therapy (HRT), except vaginal estrogen, are associated with excess risk for breast cancer.
- Excess risk is greater for estrogen+daily progesterone than estrogen-only preparations.
- Absolute risks are highest for users of estrogen+daily progesterone (1/50 users) and lowest (1/200 users) for estrogen-only preparations.
Why this matters
- The largest meta-analysis of its kind using individual-level data quantifies absolute risk and concludes that of 20 million breast cancers in western countries, 1 million have been caused by HRT since 1990.
- Meta-analysis of 24 prospective studies involving 108,647 postmenopausal women with breast cancer.
- Funding: Cancer Research UK; Medical Research Council.
- Every HRT type, except vaginal estrogen, was associated with excess breast cancer risk.
- Risk was greater with longer use, and use of estrogen preparations with daily vs intermittent progesterone.
- Excess risks for current users with only 1-4 years of use was increased by 60% for estrogen-progesterone HRT (adjusted risk ratio vs nonuse, 1.60; 95% CI, 1.52-1.69).
- Absolute breast cancer risks at ages 50-69 years for an average-weight woman in developed countries with 5 years of HRT use starting at age 50:
- Estrogen+daily progesterone: 1/50 users.
- Estrogen+intermittent progesterone: 1/70 users.
- Estrogen only: 1/200 users.
- Observational design.