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’re 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.

Prevention drugs aren’t advised for women at average risk of breast cancer. Those 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 Dr. 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.

Subjects 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 meant a 65 percent reduction in risk for those on the drug.

Still, it could be a tough sell. These women have only about a 2 percent to 3 percent risk, individually, of being diagnosed with breast cancer in the next five years. A prevention pill that cuts that chance by 65 percent means they’ll have about a 1 percent risk, said Dr. Eric Winer, breast cancer chief at Dana-Farber Cancer Institute in Boston.

“Many women are not willing to take a daily medicine” for such a small risk, he said.