Estrogen adds one more reason for concern for women who took it during menopause, as new research shows that they have more than twice the risk of developing specific types of benign breast disease, or benign breast lumps.

According to Tom Rohan, MBBS, Ph.D., of Albert Einstein College of Medicine, who led the study, benign breast disease is believed to be an early stage in the development of breast cancer and may increase the risk of later carcinoma, but this might take “a decade or so.”

This is the first study on the effect of conjugates equine estrogen (CEE), a type of hormone replacement therapy.

Dr. Rohan analyzed data coming form the Women’s Health Initiative CEE trial, in which 10,739 postmenopausal women were randomly assigned to receive CEE or a placebo. The women were followed for almost seven years.

It is already known that a combined hormone therapy estrogen plus progestin increases the risk of breast cancer and other serious conditions, but women able to use estrogen alone (meaning they previously had hysterectomies) didn’t show an increase in breast cancer risk.

The study identified 232 women who had biopsies for what turned out to be non-cancerous breast disease. These women had taken either estrogen alone or a placebo. There were 155 cases of non-cancerous proliferative breast disease in women who had taken estrogen and 77 in the placebo group, which means that women who were given estrogen-only therapy were more than twice likely to develop benign breast disease compared directly to women who were given a placebo.

“Each and every person who is a candidate for taking hormone therapy really needs to weigh the risks and benefits. You might say, this is one additional risk,” Dr. Rohan told Reuters in a telephone interview.

The study, supported by the National Heart, Lung and Blood Institute and the National Cancer Institute, was published online April 8 in the Journal of the National Cancer Institute.