Bisphosphonates are medicines that help prevent bone loss and make bones stronger. Bisphosphonates are usually prescribed for postmenopausal women.
Two studies suggest that postmenopausal women who took bisphosphonate pills by mouth were 30% to 40% less likely to develop invasive breast cancer compared to women who didn't take oral bisphosphonates.
Oral bisphosphonates are:
- Fosamax (chemical name: alendronate sodium)
- Actonel (chemical name: risedronate)
- Boniva (chemical name: ibandronate)
In the first study, researchers looked at the medical records of more than 154,000 women who are part of the on-going Women's Health Initiative (WHI) study. They compared the histories of 2,816 women taking an oral bisphosphonate when they enrolled in the WHI to the other women in the study who weren't taking an oral bisphosphonate.
After about 7.8 years, women who did take an oral bisphosphonate were:
- 30% less likely to be diagnosed with invasive hormone-receptor-positive breast cancer; this reduction in risk was statistically significant, which means it was likely because the women were taking the bisphosphonate and not just due to chance
- 34% less likely to be diagnosed with invasive hormone-receptor-negative breast cancer; this reduction in risk wasn't statistically significant, which means it could have been due chance and not because the women were taking a bisphosphonate
- 58% more likely to be diagnosed with non-invasive ductal carcinoma in situ (DCIS); this increase in DCIS risk was statistically significant, which means it was likely because the women were taking the bisphosphonate and not just due to chance
compared to women who weren't taking a bisphosphonate.
The increase in DCIS risk is surprising. This unexpected result might have happened because the bisphosphonate slowed the DCIS from developing into invasive cancer. So fewer cases of invasive cancer were diagnosed but more non-invasive DCIS cases were diagnosed.
The second study looked at breast cancer risk in 4,039 postmenopausal women, some of whom took an oral bisphosphonate. Women who took an oral bisphosphonate for at least 1 year had a 39% lower risk of invasive breast cancer. These results are comparable to the results from the WHI study.
While oral bisphosphonates can be taken for a number of years to strengthen bones, these two studies found that oral bisphosphonates reduced breast cancer risk mostly during the first year women took them and not so much in the following years.
Researchers aren't sure how bisphosphonates lower the risk of invasive breast cancer. Other research done in the lab suggests that bisphosphonates may stop cancer cells from releasing proteins that help them grow. Bisphosphonates also may block the growth of blood vessels (called angiogenesis inhibition) that give cancer cells the oxygen and nutrients they need to thrive.
These two studies looked at oral bisphosphonates. Other forms of bisphosphonates are given intravenously:
- Aredia (chemical name: pamidronate)
- Bonefos (chemical name: clodronate)
- Boniva (chemical name: ibandronate)
- Reclast and Zometa (chemical name: zoledronic acid)
Intravenous bisphosphonates weren't included in the studies reviewed here, but it's likely that they also can lower breast cancer risk.
While these results are promising, more research is needed to confirm the risk-reduction benefits of bisphosphonates. Still, if you're a postmenopausal woman, your doctor may recommend a bisphosphonate to strengthen your bones or treat osteoporosis. These results add to evidence suggesting that a bisphosphonate may lower your risk of invasive breast cancer in addition to strengthening your bones. Bisphosphonates need to be taken in a specific way and may cause serious side effects. If you're prescribed a bisphosphonate, make sure you and your doctor talk about how to take the medicine.
Editor's note: In August 2014, two randomized studies found that 3 to 4 years of treatment with either Fosamax (chemical name: alendronate sodium) or intravenous zoledronic acid doesn’t seem to reduce breast cancer risk in postmenopausal women who’ve been diagnosed with osteoporosis. While these results are disappointing, the researchers said that more studies looking at breast cancer risk and bisphosphonates in women who don’t have osteoporosis should be done. It may be that bisphosphonates help reduce risk in women with strong bones.