Earlier this month, women at risk of developing breast cancer received a limited reason for hope: A drug called tamoxifen was found effective at preventing breast cancer, but it is associated with increased rates of uterine cancer.Now comes a potentially bigger break-through. As yet unreleased results of clinical trials of an Eli Lilly & Co. drug called Evista indicate that it, too, is effective at preventing breast cancer - without any increase in the risk of uterine cancer.