Hormone replacement therapy, known to increase the risk of breast cancer, was also discovered to increase risk of recurrence of breast cancer, according to a new study.

Researchers at King’s College London and Scandinavia revealed that women who earlier had breast cancer were 14 percent more likely to get it again if they followed HRT. “The results…indicate a substantial risk for a new breast cancer event among breast cancer survivors using hormone-replacement therapy,” wrote researcher Lars Holmberg, M.D., Ph.D. and colleagues at Kings College London.

The study followed 442 Scandinavian women who had breast cancer over a period of four years. Half of the women received hormone-replacement therapy. The findings showed a 17.6 percent risk of recurrence or a new tumor in the HRT group, compared with a 7.7 percent risk among women in the control group. The estimated five-year risk for a recurrence was 22.2 percent for women in the HRT group and 9.5 percent in the control arm, translating into a 14.2 percent absolute increased risk for women taking hormones.

"Our results further suggest that hormone therapy not only induces and promotes breast cancer but may also stimulate the growth of tumor microdeposits in breast cancer survivors," the researchers wrote.

They concluded that physicians and their patients should continue to weigh the risks and benefits of HRT in postmenopausal breast cancer survivors.

In an editorial commentary, Dr. Kathleen Pritchard, M.D., a breast cancer specialist at Sunnybrook Odette Cancer Center in Toronto welcomed the findings of the study saying the study “suggests quite definitely that there is a statistically significant increased risk of recurrence in women given HRT following diagnosis of breast cancer. It seems that the harmful side effects of HRT have finally been clearly demonstrated…Why did we wait so long?”

Almost 25 million American women use hormone replacement therapy to alleviate the discomforts of menopause, including hot flashes, mood swings, and night sweats. The number is dropping since several studies have revealed the risks women are exposed to once taking the treatment. Since 2002, sales of Wyeth’s Prempro have dropped from $2 billion a year to a little more than $1 billion. Women went off their pills, choosing to risk hot flashes and brittle bones rather than heart attacks and breast cancer.

The findings of the study appeared in the March 25 online issue of the Journal of the National Cancer Institute.