TIME Breast Cancer

Osteoporosis Drugs Do Not Prevent Breast Cancer After All

Some studies had hinted that the bone-building treatments may also have an added benefit in fighting tumors, but the latest study doesn’t support that connection

In recent years, several large studies involving tens of thousands of women found a potentially useful connection between bisphosphonates, the popular bone drugs, and a lower risk of breast cancer. But new research published Monday in JAMA Internal Medicine challenges that long-held belief.

In previous observational studies, women who reported taking medications like alendronate (Fosamax) or zoledronic acid (Reclast) to treat osteoporosis also seemed to have lower rates of breast cancer compared to women who didn’t take the medications. There was biological evidence to support the association as well – bisphosphonates were also correlated with lower rates of cell death and hampered cancer cell activity.

But Trisha Hue, an epidemiologist at the University of California, San Francisco, and her colleagues, wondered if the connection could truly be attributed to the osteoporosis medications, or whether there was something else about the women taking these drugs that could explain the cancer trend.

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Indeed, when they focused their attention on two studies that randomly assigned women to either a bisphosphonate or placebo, and followed them for up to four years, they found no difference between the women taking the drug and those who did not when it came to their breast cancer rates.

So why the strong connection in previous studies? Hue points out that those analyses, which were not randomized controlled trials, but rather observational studies, could not account for the fact that the drug-taking group may have been biased in some way. And in fact that’s likely what occurred – women who are prescribed bisphosphonates have low bone mineral density, and they also have low levels of the hormone estrogen, which is known to fuel tumor growth. So the earlier studies were not finding a correlation between bisphosphonate use and a lower risk of breast cancer, but instead were picking up the fact that bisphosphonate users were likely to have lower rates of breast cancer to begin with.

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It’s not the first time the benefits of the bone drugs have been called into question. In 2011, some studies found that the therapies could increase the risk of rare bone fractures in the strong bones such as the thigh.

While some doctors and patients may have turned to the bone-building drugs to potentially avoid getting cancer, Hue says “our take-home message is that if you are already on bisphosphonates for prevention of fractures, it’s very effective for preventing fractures. But they shouldn’t be taken specifically for the primary prevention of breast cancer.”

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