Chemotherapy drugs are given either before the surgery to shrink the tumor (neo-adjuvant therapy), or after the surgery, to keep the cancer from growing back (adjuvant therapy.)
This approach involves the use of systemic agents to lower a patient’s risk of developing cancer by blocking the body’s natural production of hormones. Tamoxifen is approved for use in prevention, but is associated with serious side effects (including hot flashes, increased risk of uterine cancer, and blood clots). A similar drug, raloxifene, has fewer side effects and was recently approved by the FDA to reduce risk of breast cancer.
This type of treatment attacks specific types of cancer cells with less harm to normal cells. Some types of targeted therapies help the immune system kill cancer cells or deliver toxic substances directly to cancer cells and kill them. These are called immunotherapy drugs.
Other targeted therapies block the action of certain proteins involved in the growth and spread of cancer cells. These are called monoclonal antibodies.
Targeted therapy may have fewer side effects than other types of cancer treatment.
More information can be found here http://cancer.columbia.edu/breast-cancer