Assessing the risk of breast cancer is extremely difficult, except for women who've already contracted cancer in one breast -- the category of patient on which the tamoxifen study was conducted. The National Cancer Institute offers some guidelines and a "risk disk" of software to guide women in assessing the danger. "Gather all the available evidence, carefully assess your risk and talk to your doctor and any others who can help you decide," advises Gorman. "This is not a decision you should make on your own."
The first drug approved by the FDA for cancer prevention may be a bitter pill for many. Tamoxifen substantially reduces the instance of breast cancer in high-risk women, but it also doubles the risk of uterine cancer and trebles the risk of potentially fatal blood clots. "The decision to use tamoxifen if you're not yet sick is a really tricky one," says TIME science correspondent Christine Gorman. "You have to figure out whether you are truly at sufficient risk to warrant taking a potentially dangerous drug."