Exactly how the drugs reduce risk isn't known. Chlebowski speculated that the drugs may block the release of growth factors that would encourage tumors to grow or may block blood vessel formation within a tumor.
It's known that low bone mineral density (BMD) is linked with a reduced risk of breast cancer, and women with low BMD are likely to be on the drugs. So for the study analysis, Chlebowski adjusted for this possible confounding effect by incorporating a hip fracture risk score to take into account the bone mineral differences between drug users and non-users.
Another expert, Dr. Joanne Mortimer, director of the women's cancers program at the City of Hope Comprehensive Cancer Center in Duarte, Calif., pointed out that the studies found an associative link, not a cause-and-effect, so it's not definitive.
However, she said, "for people with osteoporosis, it's one more reason to feel comfortable taking a bisphosphonate."
Like other medications, the drugs have favorable and unfavorable effects. For instance, researchers recently found women on the bone-building drugs can have a higher risk of an uncommon fracture; that research is being evaluated further, Mortimer said.
>From the two studies, however, Mortimer said, it appears that "these drugs change the environment in such a way that cancer cells are less likely to take root and grow, not only in the bone marrow but elsewhere as well."
In an accompanying editorial, Dr. Michael Gnant, of the Medical University of Vienna, said future studies will help pinpoint the benefit of the drugs in breast cancer incidence reduction and supply more answers as to their best use.
Chlebowski reported that he has been a consultant to Novartis and Amgen, which make the bone-building medications.
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