Postmenopausal women undergoing a common estrogen replacement regimen have more than twice the risk of developing benign breast disease than women receiving no treatment, according to a new study.

The study, posted in the online edition of The Journal of the National Cancer Institute, tracked some 10,739 postmenopausal women.

Some of the subjects in the study were given conjugated equine estrogen, a commonly-prescribed form of estrogen, while others received a placebo. After a median follow-up of seven years, the researchers found a total of 232 cases of benign proliferative breast disease – 155 among the women who took estrogen supplements, and about half as many, 77, in the placebo group.