A new study reveals that women who take combined hormone therapy for about five years have a higher risk of abnormal mammograms and breast biopsies.

Almost 25 million American women use hormone replacement therapy to alleviate the discomforts of menopause, including hot flashes, mood swings, and night sweats.

Rowan Chlebowski, a doctor at the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center, and his colleagues tracked 16,608 post-menopausal women who participated in a five-year clinical trial called the Women’s Health Initiative, which included researchers from Kaiser’s Center for Health Research in northwest Portland.

About half of the group, meaning 8,506 women, received hormone therapy consisting of estrogen and progesterone, while the other half, meaning 8,102 women, received a placebo.

Over the five-year study, 199 women in the hormone group and 150 women in the placebo group developed breast cancer. Women taking hormones were likely to have abnormal mammograms results (35 percent versus 23 percent) and breast biopsies (10 percent versus 6.1 percent) than women taking placebos.

The researchers do not have a clear answer why mammograms are showing more abnormalities, but it may be due to the increase in breast density caused by taking hormones. That is why more study is needed in this area.

“We should also ask whether we should be developing more imaging technologies” beside the mammogram, Chlebowski said.

The findings should prompt researchers to look at better tool to diagnose breast cancer, Dr. Chlebowski said.

“This is really definite evidence that hormones interfere with breast cancer diagnosis. We have other imaging techniques, and this could be a signal that we need more of that research.”

As a response to the findings of the study, Wyeth Pharmaceuticals, which makes the hormonal product Prempro, released a statement saying “while the [study] authors report a link between an increase in abnormal mammograms and breast density among women taking combined estrogen plus progestin, this does not mean they are at an increased risk for breast cancer. The data used in this study sub-analysis were taken from the combined estrogen plus progestin database of the Women’s Health Initiative study and does not reflect the experience of the majority of women taking hormone therapy – those who take estrogen-alone,” the statement said, according to Forbes.

However, in a prepared statement, Chlebowski said, “these findings represent a concern for post-menopausal women who are considering hormone therapy. They should take the results of this study into consideration and consult with their physicians before undergoing even short-term hormone therapy,” Reuters reported.

Women over 40 are recommended to get an annual mammogram, the standard imaging test for breast cancer, one of the most common and deadly forms of the disease. An estimated of 12.7 percent of women in the U.S. will be diagnosed with breast cancer at some point in their lives, according to the National Cancer Institute.

The findings appeared in Monday issue of the Archives of Internal Medicine, a scholarly publication of the Journal of American Medical.