Women who already were using these medicines when the study began were about one-third less likely to develop invasive breast cancer over the next seven years than women not taking such pills, doctors reported Thursday.

The study is not enough to prove that these drugs, called bisphosphonates, prevent cancer. More definitive studies should give a clearer answer in a year or two.

Yet it greatly amplifies the hopeful buzz that started last year when researchers reported that a bisphosphonate cut the chances that cancer would come back in women already treated for the disease.

"Now we're actually looking at this in the general population — healthy women who have never had breast cancer. And it looks like it's protective in those women as well," said Dr. Peter Ravdin of the University of Texas Health Science Center at San Antonio.

"There's a strengthening story here," said Ravdin, who helped review the research for the San Antonio Breast Cancer Symposium, where results were reported Thursday. "This is very promising."

Millions of women already take bisphosphonates for bone-thinning osteoporosis, or to prevent fractures from cancer that has spread to their bones.