What did the Women's Health Initiative actually show?

The Women's Health Initiative was two separate randomised trials, not one study. It enrolled 16,608 women with a uterus to conjugated equine estrogens 0.625 mg plus medroxyprogesterone acetate 2.5 mg or placebo, and 10,739 women without a uterus to conjugated equine estrogens alone or placebo. The average age at enrolment was 63. The question was whether hormones prevent chronic disease in older women, not whether they relieve hot flashes at 51.

What the estrogen plus progestin arm showed

The estrogen plus progestin arm was stopped early in 2002. Rossouw and colleagues reported in JAMA an excess of 8 invasive breast cancers, 7 coronary events, 8 strokes and 8 pulmonary emboli per 10,000 women per year, against 6 fewer colorectal cancers and 5 fewer hip fractures. The headline that travelled was a 26 percent increase in breast cancer. In absolute terms that was fewer than one extra case per 1,000 women each year, a figure that almost never appeared alongside the percentage.

What the estrogen alone arm showed

The estrogen alone arm gave a different answer, and this is the part most often lost. In the 2013 JAMA analysis by Manson and colleagues, conjugated equine estrogens alone produced 8 extra strokes and 7 fewer hip fractures per 10,000 women per year during the treatment phase. Invasive breast cancer did not increase, with a hazard ratio of 0.79 and a confidence interval from 0.61 to 1.02. Coronary heart disease did not increase, with a hazard ratio of 0.94. All-cause death was unchanged in both arms.

What age did to the numbers

Age changed the balance sheet. In the same 2013 analysis, women aged 50 to 59 on estrogen plus progestin had 12 more adverse events per 10,000 women per year on the trial's combined global index, while women aged 50 to 59 on estrogen alone had 19 fewer. The FDA cited this age problem on 10 November 2025 when it removed the boxed warning on cardiovascular disease, breast cancer and dementia, noting the average participant was over a decade past the average age of menopause.

The trial never tested transdermal estradiol, micronised progesterone, or low doses, and it was not designed to answer what happens to a symptomatic 52 year old who starts a patch. Ask your clinician for absolute numbers rather than percentages, and ask which arm of the trial the number you are being quoted actually came from.