It seems like there has been a flood of recent studies questioning the safety of hormone replacement therapy (HRT). Some large-scale studies have found that HRT increases the risk of certain types of breast cancer, not to mention stroke and heart attack; others have found that HRT may cause the breast tissue to become denser, making mammograms harder for doctors to interpret. But does the timing of HRT affect its safety? Current research shows that it may.

For women who began HRT less than five years after menopause began, the risk of developing breast cancer was 43% greater than women who had never used it. The risk was even higher for women who had used estrogen-progesterone combination therapy.

In the new study, researchers studied over 1 million menopausal women (average age: 56) who took part in the Million Women Study in Great Britain. The researchers tracked whether or not the women developed breast cancer over the course of the study, whether they had taken HRT, at what age they had begun it, how long they had taken the hormones, and what kind of hormones they had used (estrogen only or estrogen-progesterone combination).

There was a strong relationship between the age a woman began HRT and her risk for developing breast cancer. For women who had waited to begin estrogen-only HRT for at least five years after the onset of menopause, their risk was no different from women who had never used HRT. But for women who began HRT less than five years after menopause began, the risk of developing breast cancer was 43% greater than women who had never used it. The risk was even higher for women who had used estrogen-progesterone combination therapy.

These findings may be worrisome to those who are currently on HRT, but the team also found some encouraging results, which confirmed earlier findings. When women in the study stopped using HRT, their risk for developing breast cancer returned to that of "never users" within just a few years — and it stayed at this lower level for the entire length of time the women were followed, which was about 14 years. This means that HRT does not raise the risk of breast cancer forever, if one stops using it.

The symptoms of menopause can be extremely unpleasant for some women, and for these women, HRT often brings much-needed relief. But as more studies are showing, it comes with some serious risks, not only to the breasts but to other parts of the body as well. For women who are already on HRT or who are considering it, it is important to talk with your doctor about your specific circumstances, and all the risks and benefits involved.

This research was led by researchers at the University of Oxford and published in the January 28, 2011 online issue of the Journal of the National Cancer Institute. It and can be accessed here.