CHICAGO - Millions of women at higher-than-usual risk of breast cancer have a new option for preventing the disease. Pfizer Inc.'s Aromasin cut the risk of developing breast cancer by more than half, without the side effects that have curbed enthusiasm for other prevention drugs, a major study found.

It was the first test in healthy women of newer hormone-blocking pills called aromatase inhibitors, sold as Arimidex, Femara, and Aromasin, and in generic form. They are used now to prevent recurrences in breast-cancer patients who are past menopause, and doctors have long suspected they may help prevent initial cases, too.

Women at higher risk because of gene mutations or other reasons already have two choices for prevention - tamoxifen and raloxifene. But these drugs are unpopular because they carry small risks of uterine cancer, blood clots, and other problems.

"Here's a third breast-cancer-prevention drug that may in fact be safer," said Allen Lichter, chief executive of the American Society of Clinical Oncology.

The study was discussed Saturday at the society's annual meeting in Chicago, along with another one that could change care for thousands of women each year with breast cancer that has spread to lymph nodes. It found that giving radiation to the armpit - not just the breast - after surgery significantly lowered the chances the cancer would come back.

The prevention study involved 4,560 women from the United States, Canada, Spain, and France. They had at least one risk factor - being 60 or older, a previous breast abnormality or pre-invasive cancer, or a high score on a scale that takes into account family history and other factors.

They were given daily doses of exemestane, sold as Aromasin, or dummy pills. After about three years, there were 11 cases of invasive breast cancer among those on the drug versus 32 among the others. That worked out to a 65 percent reduction in risk for those on the drug - enough of a benefit that independent monitors decided all participants should be offered it.

Serious side effects, such as broken bones, high cholesterol and heart problems, were similar in both groups. Slightly more women on the drug reported hot flashes, fatigue, sweating, insomnia and joint pain, but quality-of-life scores were similar.

Earlier studies of aromatase inhibitors found they can cause bone loss, vaginal dryness, problems having sex, joint pain and muscle aches, so it will take longer study to see if these occur, Lichter said.

Worldwide, about 1.3 million women are found to have breast cancer each year, and nearly 500,000 women die of the disease. Last year in the United States, there were about 207,000 new cases and 40,000 deaths from breast cancer.