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The Women's Health Initiative (WHI), an NIH-sponsored, multicenter effort begun in 1993, has been conducting 2 randomized, placebo-controlled trials of hormone therapy (HT) in postmenopausal women: an estrogen-only study for women who have had a hysterectomy (n=10,739) and an estrogen/progestin (E+P) study for women with a uterus (n=16,608). The active treatment regimen in the E+P study was 0.625 mg of conjugated equine estrogen plus 2.5 mg of medroxyprogesterone acetate daily.
On July 9, 2002, the NIH announced that the E+P study was being halted after a mean of 5.2 years of follow-up, rather than the planned 8.5 years, because those on active treatment had an increased risk for breast cancer in excess of the safety threshold and had an inc…