Hormone replacement therapy (HRT) may trigger breast cancer, according to research published in the journal Archives of Internal Medicine.
The study found that women who suffered breast tenderness as a result of HRT treatment were 48% more likely to suffer from invasive cancer.
It also found women given oestrogen plus progestin HRT were three times more likely to develop the tenderness after a year, compared to those who took placebos.
All told, the trial found that HRT increased the risk of breast cancer by 26%, heart attacks by 29% and strokes by 41%. Blood-clot risk was doubled, and cardiovascular risk increased by 22%.
As a result of the findings, the trial, which had been due to run for eight years, was abandoned after five.
Says lead researcher Dr Carolyn Crandall, of the University of California, Los Angeles: "We need to figure out what makes certain women more susceptible to developing breast tenderness during hormone therapy than other women."
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