The Women's Health Initiative study linked HRT to breast cancer, but the culprit was the synthetic progestin, not estrogen. Estrogen alone actually shows lower breast cancer rates. The misconception persists even though later research clarified what the original study got wrong.
I want to clarify that hormones do not cause breast cancer. So a lot of what we know about hormones and breast cancer came from this large study that people still quote called the Women's Health Initiative. It was the largest NIH-sponsored trial designed to see whether the use of hormones prevented heart disease and whether it was associated with any adverse risks. And there was a lot of bad publicity when part of the study was stopped short with the finding it didn't prevent against heart disease. and it surpassed the threshold for breast cancer. But in that study, they used specific types of hormones, the ones that were most commonly prescribed when they were accruing patients for that study, and they were synthetic hormones, and in particular, a synthetic progestin. So when women take hormones, estrogen is the component that tends to make you feel good. But if you have a uterus, you need to balance that with some form of a progestogen to protect against uterine cancer. In Women's Health Initiative, they used a synthetic form of a progestogen. And studies since then have confirmed that largely the risk of breast cancer is associated with combination estrogen and synthetic forms of progestins. Turns out estrogen alone is actually associated with lower risk of getting and lower risk of dying of breast cancer. So we know it's more the progestogen component and specifically these synthetic progestins.
