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A drop in the popularity of hormone replacement therapy (HRT) has led to a concurrent drop in rates of breast cancer, according to a study conducted by researchers from Stanford University and published in the New England Journal of Medicine.
"This is very strong evidence that estrogen plus progestin causes breast cancer," said researcher Marcia Stefanik. "You start women on hormones and within five years their risk of breast cancer is clearly elevated. You stop the hormones and within one year their risk is essentially back to normal. It's reasonably convincing cause-and-effect data."
The researchers monitored breast cancer rates among 15,000 women who had participated in the original, groundbreaking 2002 Women's Health Initiative study that had first demonstrated a link between combined HRT and increased breast cancer risk. All participants in that study had been encouraged to stop using the therapy. These women were then compared with another group that had never been told to stop using HRT.
The risk of breast cancer among women in the Women's Health Initiative group was highest in the five years before 2002, when all of them were using HRT. After the women stopped using the therapy, breast cancer rates plummeted quickly, leading to 28 percent fewer diagnoses in the next year.
Some skeptics of an HRT-cancer link have suggested that falling rates of cancer diagnosis since the Women's Health Initiative study are caused not by women abandoning HRT, but by a drop in the rate of mammograms. But the researchers in the current study found that the frequency of mammograms among former Women's Health Initiatives did not change, while the caner rate did.
Among women who had not participated in the first study, half chose to stop using HRT. This led to a 43 percent reduction in the rates of cancer diagnosis among that group. Women who continued taking HRT for another five years, however, experienced a doubling of their breast cancer risk every year.
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