It is the first large study to affirm wider anti-cancer hopes for Zometa and other bone-building drugs called bisphosphonates. Zometa, made by Novartis AG, is used now for cancers that have already spread to the bone.
The new study involved 1,800 premenopausal women taking hormone treatments for early-stage breast cancer.
Zometa cut by one-third the chances that cancer would recur -- in their bones or anywhere else.
"This is an important finding. It may well change practice," said Dr. Claudine Isaacs, the director of the clinical breast cancer program at Georgetown University's Lombardi Cancer Center.
About three-fourths of breast cancers occur in women after menopause. Zometa might help them, too, but it hasn't been tested yet in that age group.
The study was led by Dr. Michael Gnant, of the Medical University of Vienna, and reported Saturday at an American Society of Clinical Oncology conference in Chicago.
If a second, ongoing study also finds a benefit, doctors predict Zometa will quickly be tested against other cancers that tend to spread, or metastasize, to bones, such as prostate and kidney cancer.
"Hugely important is whether this has to do with the fact that it just makes the bone hostile, somehow, to metastasis or if there is a more global anti-metastasis effect," said the oncology group's president, Dr. Nancy Davidson, of Johns Hopkins University. "Either of those would be good and would teach us a lot about what to do next."
Breast cancer is the most common cancer in women. About 184,450 cases and 40,930 deaths from the disease are expected in the United States this year.