CHICAGO — A drug now used to prevent recurrences of breast cancer can also reduce the risk of it occurring in the first place, providing a new option for women at high risk of getting the disease, researchers reported here on Saturday.

Two drugs, tamoxifen and raloxifene, are already approved to prevent breast cancer but both are rarely used for that purpose, in part because they can have serious side effects like blood clots. The researchers said the new option, exemestane, does not have those side effects and might be more acceptable.

“There’s a very safe therapy that looks highly effective in preventing breast cancer,” Dr. Paul E. Goss, professor of medicine at Harvard and Massachusetts General Hospital, said at a news conference at the annual meeting of the American Society of Clinical Oncology. He was the lead investigator in the study, which was presented at the conference and published online by the New England Journal of Medicine.