Does hormone replacement therapy (HRT) put me at risk for breast cancer?

The Women's Health Initiative and a series of randomized controlled studies looked at large numbers of women. Women who were similar were randomly enrolled into one of two groups. One group received hormone replacement therapy (HRT), and one group received a placebo.

In the Women's Health Initiative, the group given HRT was terminated from study before the end of the study. Not because HRT was harmful to some, but, even worse, because no benefit could be found and HRT was found to be harmful. Yes, the risk of breast cancer was higher in the patients undergoing estrogen and progestin (a progesterone) replacement therapy: four more breast cancers every 5,200 years of treatment. That is to say, if 5,200 people were treated for one year, there would be an increase of four breast cancers. Here's another way of looking at it: If 1,125 people were treated for four years, there would be an increased risk of four breast cancers for that group. (Perhaps it would be more relevant to say that if 225 people were treated for twenty years, four more would develop breast cancer.) There was also an increase in heart attacks of four in 5,200 and an increase in strokes of four in 5,200, but no difference in mortality (deaths) between the women given HRT and those not given HRT.