Breast Cancer Health Center

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Major inheritance susceptibility

Based on solid evidence, women who inherit gene mutations associated with breast cancer have an increased risk.

Magnitude of Effect: Variable, depending on gene mutation, family history, and other risk factors affecting gene expression.

Factors Associated With Decreased Risk of Breast Cancer

Exercise

Based on solid evidence, exercising strenuously for more than 4 hours per week is associated with reduced breast cancer risk.

Magnitude of Effect: Average RR reduction is 30% to 40%. The effect may be greatest for premenopausal women of normal or low body weight.

Early pregnancy

Based on solid evidence, women who have a full-term pregnancy before age 20 years have decreased breast cancer risk.

Magnitude of Effect: 50% decrease in breast cancer compared with nulliparous women or those who give birth after age 35 years.

Breast-feeding

Based on solid evidence, women who breast-feed have a decreased risk of breast cancer.

Magnitude of Effect: The RR of breast cancer is decreased 4.3% for every 12 months of breast-feeding, in addition to 7% for each birth.[1]

Interventions Associated With Decreased Risk of Breast Cancer

Selective estrogen receptor modulators (SERMs): Benefits

Based on solid evidence for tamoxifen and fair evidence for raloxifene, treatment reduces the incidence of breast cancer in postmenopausal women. Tamoxifen also reduced the risk of breast cancer in high-risk premenopausal women. The effects observed for tamoxifen show persistence several years after discontinuing active treatment.

Magnitude of Effect: Treatment with tamoxifen-reduced breast cancer by about 50%. Treatment with raloxifene has a similar effect on reduction of invasive breast cancer but appears to be less effective for prevention of noninvasive tumors.

Selective estrogen receptor modulators (SERMs): Harms

Based on solid evidence, tamoxifen treatment increases the risk of endometrial cancer, thrombotic vascular events (pulmonary embolism, stroke, deep venous thrombosis), and cataracts. Many of these risks, notably pulmonary embolism and deep venous thrombosis, are reduced after discontinuing active treatment with tamoxifen. Based on fair evidence, raloxifene also increases venous pulmonary embolism and deep venous thrombosis but not endometrial cancer.

Magnitude of Effect: Meta-analysis shows RR = 2.4 (95% CI, 1.5-4.0) for endometrial cancer and 1.9 (95% CI, 1.4-2.6) for venous thromboembolic events.

Aromatase inhibitors or inactivators: Benefits

Based on fair evidence, aromatase inhibitors or inactivators (AIs) reduce the incidence of new breast cancers in postmenopausal women who have a history of breast cancer.