From the National Cancer Institute at the National Institutes of Health:
Breast Cancer Prevention (By clicking each factor below on the NIH web page linked, detailed information is shown on that factor)
Key Points for This Section
Avoiding risk factors and increasing protective factors may help prevent cancer.
The following risk factors may increase the risk of breast cancer:
Estrogen (endogenous)
Hormone replacement therapy/Hormone therapy
Exposure to Radiation
Obesity
Alcohol
Inherited Risk
The following protective factors may decrease the risk of breast cancer:
Exercise
Estrogen (decreased exposure)
Selective estrogen receptor modulators
Aromatase inhibitors
Prophylactic mastectomy
Prophylactic oophorectomy
Fenretinide
The following have been proven not to be risk factors for breast cancer or their effects on breast cancer risk are not known:
Abortion
Oral Contraceptives
Environment
Diet
Active and passive cigarette smoking
Statins
Cancer prevention clinical trials are used to study ways to prevent cancer.
New ways to prevent breast cancer are being studied in clinical trials.
Breast Cancer Prevention (PDQ®) - National Cancer Institute