Research · PMID 12927427 · Landmark study
Reviewed by Neal Rouzier, MD, Preventive Medicine · Sep 2026
Breast cancer and hormone-replacement therapy in the Million Women Study.
Lancet (London, England) · August 2003 · Valerie Beral, Million Women Study Collaborators
What this study found
Current use of hormone-replacement therapy (HRT) increases the incidence of breast cancer. The Million Women Study was set up to investigate the effects of specific types of HRT on incident and fatal breast cancer.
Why this trial matters
Million Women Study HRT breast cancer
How RAGMD uses this study
Clinical questions this paper informs
Synthesis lives on the evidence question — this page shows where the study sits in that analysis.
Conflicting evidence
Does hormone replacement therapy increase breast cancer risk?
Combined estrogen-progestin therapy is associated with a small increased breast cancer risk in WHI and observational data. Estrogen-alone in hysterectomized women showed lower risk in WHI, but interpretation depends on regimen, duration, and individual risk factors.
Open evidence analysisConflicting evidence
Is progestogen required to protect the endometrium with estrogen therapy?
Systemic estrogen in women with a uterus requires adequate progestogen exposure to prevent endometrial hyperplasia. Micronized progesterone and progestins differ in metabolic and VTE profiles.
Open evidence analysis