BAKERSFIELD, Calif. -- Millions of women take bone-building drugs called bisphosphonates to fight osteoporosis, but new research finds these medications may also reduce the risk for breast cancer.

Doctors from the Fred Hutchinson Cancer Research Center examined health records for nearly 6,000 women -- half of whom had breast cancer.

Those who took osteoporosis drugs such as Fosamax, Boniva and Zometa for more than two years cut their breast cancer risk by almost 40 percent. But the benefits were only seen in women who were not obese.

Researchers speculated that higher levels of estrogen in obese women may counteract any effects of the bone-strengthening drugs.

How these medications may reduce cancer risk is not yet known, but preliminary evidence has suggested they could help cancer cells self-destruct or keep cells from sticking together to form tumors.

Doctors said women should not yet start osteoporosis drugs simply to lower breast cancer risk because more confirmatory research is needed -- but for those who are already taking the medications, it's additional motivation to stick with your treatment.