Osteoporosis drugs known as bisphosphonates may not protect women from breast cancer as had been thought, according to a new study. The drugs’ protective effect was widely assumed after several observational studies showed that women who took them were less likely to develop breast cancer.

But when researchers, led by those at the University of California San Francisco (UCSF), assessed the effect of two of the most widely used osteoporosis drugs, which are sold under the brand names Fosamax and Reclast, in two large randomized clinical trials, neither drug protected women with osteoporosis from developing breast cancer. The results were published in JAMA Internal Medicine (2014; doi:10.1001/jamainternmed.2014.3634).

The researchers explained the link between bisphosphonates and a lower incidence of breast cancer that was seen in previous observational studies may be due to a third factor: low estrogen. Since having low estrogen both weakens bones and protects against most breast cancers, the women most likely to be prescribed drugs for osteoporosis are usually also at lower risk for breast cancer.

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“They may have seen a lower risk of breast cancer in women using bisphosphonates in the earlier observational studies because those women had a lower risk of breast cancer to begin with,” said lead author Trisha Hue, PhD, of UCSF. “Postmenopausal women with osteoporosis usually have low estrogen levels. Lower levels of estrogen are strongly associated with a lower risk of breast cancer.”

Steven Cummings, MD, a co-author of the paper, a UCSF emeritus professor of medicine and director of the San Francisco Coordinating Center, said women should not take these drugs thinking they will protect them from breast cancer.

“Postmenopausal women should continue taking these drugs for the prevention of fractures, but they should not use bisphosphonates for the prevention of primary breast cancer,” said Cummings.

The data analyzed in the study came from two double-blind, placebo-controlled clinical trials. In both trials, women who received bisphosphonates had a slightly higher but statistically nonsignificant incidence of breast cancer.

The Fracture Intervention Trial (FIT) randomly assigned 6,459 women from the United States age 55 to 81 years to alendronate or a placebo for an average follow up of 3.8 years; 1.8% of the women who received the drug developed breast cancer, whereas 1.5% of those given a placebo developed the disease.

The Health Outcomes and Reduced Incidence with Zoledronic Acid Once Yearly-Pivotal Fracture Trial (HORIZON-PFT) randomly assigned 7,765 women from the United States, Canada, Asia, Europe, and South America age 65 to 89 years to annual intravenous zoledronic acid or a placebo for an average follow-up of 2.8 years; 0.87% of women who received the drug developed breast cancer, whereas 0.77% of those given a placebo developed the disease.

Women who had recurrent breast cancer or reported a history of breast cancer were excluded from the analysis in both studies.