Women who took a commonly used class of osteoporosis drugs called bisphosphonates had significantly fewer invasive breast cancers than women not using the bone-strengthening pills, according to a new analysis of data from the Women's Health Initiative (WHI).

The analysis from a segment of the more than 150,000 generally healthy post-menopausal women in the WHI study found that those taking Merck & Co's Fosamax, or other bisphosphonates, had 32 percent fewer cases of invasive breast cancer than women who did not use the osteoporosis medicines, researchers found.

Fosamax is now available in generic form as alendronate. Other commonly used medicines from the class include Roche's Boniva and Actonel, which is sold by Procter & Gamble Co.

The idea that bisphosphonates could reduce breast cancer incidence is very exciting because there are about 30 million prescriptions for these agents written annually in the United States targeting bone health, and more could easily be used to counteract both osteoporosis and breast cancer, Dr. Rowan Chlebowski, the study's lead investigator and chief oncologist from the Los Angeles Biomedical Research Institute, said in a statement.

It was the landmark WHI research program that in 2002 found a link between long-term use of hormone replacement therapy by post-menopausal women and increased risk of breast cancer and heart attacks -- findings that have been used as the basis for thousands of lawsuits against the makers of those drugs.

The latest findings from the observational study were presented at the San Antonio Breast Cancer Symposium on Thursday.

The impetus for looking at the connection between bisphosphonates and breast cancer came from data from a clinical trial in which breast cancer patients who were given Novartis' bisphosphonate Zometa intravenously every six months had fewer contralateral breast cancers, Chlebowski explained.

It appeared to make bone less hospitable to breast cancer, Chlebowski said.

Contralateral breast cancer is typically a second new case of cancer in the other breast, rather than the spread of the originally detected breast cancer.

Studying 2,816 participants who were using bisphosphonates when they entered the WHI program, researchers found that only 64 women developed breast cancer. That translates into 32 percent fewer breast cancers in women using bisphosphonates compared with women who did not use them, researchers said.

Bisphosphonates reduce angiogenesis (flow of blood and nutrients to the tumor) and stimulate immune cells responsible for tumor cell surveillance as potential mediators, Chlebowski said. This association needs to be studied further.

Researchers cautioned that this was an observational study that does not necessarily carry the same scientific weight as a blinded clinical trial.

However, they said, several ongoing breast cancer trials evaluating oral and intravenous bisphosphonates will be available in the near future to provide randomized clinical trial evidence regarding their influence on new contralateral breast cancer risk.

This is not a definitive finding, Chlebowski said in an interview. But I think it could influence the decision making of women who are deciding whether to take a bisphosphonate or not.