So-called "chemoprevention" drugs are given to healthy women, not to treat breast cancer, but to reduce the risk of ever developing it. Chemoprevention drugs sound exciting. Who wouldn't want a drug that could ward off breast cancer? But given the many unanswered questions and some serious side effects, the potential harms may outweigh the potential benefits. And the research has been on high-risk women only, so we don't know anything about how these drugs would impact most women. Among many other things, we don't know if the reduction in risk is short term or if the drugs reduce the risk of dying from breast cancer.
There are special challenges to giving risk reduction drugs to healthy women. Most women will never get breast cancer, whether they take the drug or not. So only a small proportion of women taking the drug will benefit, while many others will be unnecessarily exposed to side effects and other known—and unknown—harms.