Women who took a commonly prescribed estrogen in hormone therapy have more than twice the risk of developing specific types of benign breast disease, a new study has found.

A combined hormone therapy estrogen plus progestin is already known to increase the risk of breast cancer but a major study of women able to use estrogen alone didn't find that link.

In the Women's Health Initiative study, 10,739 postmenopausal women who had undergone hysterectomies were assigned to either conjugated equine estrogen or a placebo. A total of 232 cases of benign proliferative breast disease were identified, with 155 cases among the women who took estrogen supplements and 77 in the placebo group.

Benign proliferative breast disease is a condition that is associated with increased risk of breast cancer.

The randomized controlled trial was published online April 8 in the Journal of the National Cancer Institute.