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The Women's Health Initiative (WHI) randomized trials showed that estrogen-progestin therapy (EPT) increased risk for invasive breast cancer while estrogen therapy (ET) reduced risk. Now, using two U.K. primary care databases, investigators identified some 99,000 women with breast cancer diagnosed between 1998 and 2018 (age range, 50–79; mean age at diagnosis, 63; >95% white) and matched them with 457,000 controls. Analyses were adjusted for smoking, body-mass index (BMI), ethnicity, and mammography.
Ever-use of EPT had an adjusted odds ratio for breast cancer of 1.26 (95% confidence interval, 1.24–1.29), while ET had an aOR of 1.06 (95% CI, 1.03–1.10). In women aged 50–59 who used EPT for ≥5 years, 15 additional breast cancers were diagnose…