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Breast Cancer Health Center

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    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.[3]

    Study Design: Case-control and cohort studies.
    Internal Validity: Good.
    Consistency: Good.
    External Validity: Good.


    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.

    Study Design: Prospective observational and case-control studies.
    Internal Validity: Good.
    Consistency: Good.
    External Validity: Good.

    Estrogen use by women with prior hysterectomy


    Based on fair evidence, women who have undergone a prior hysterectomy and who are treated with conjugated equine estrogen have a lower incidence of breast cancer. However, epidemiological studies yield conflicting results.

    Magnitude of effect: After 6.8 years, incidence was 23% lower in women treated with estrogen in an RCT (0.27% per year, with a median of 5.9 years of use, compared with 0.35% per year among those taking a placebo), but was 30% higher in women treated with estrogen in an observational study. The difference in these results may be explained by different screening behavior by the women in both studies.

    Study Design: One RCT, observational studies.
    Internal Validity: Fair.
    Consistency: Poor.
    External Validity: Poor.


    Based on solid evidence, women who have undergone hysterectomy and who are taking postmenopausal estrogen have an increased risk of stroke and total cardiovascular disease.

    Magnitude of Effect: There is a 39% increase in the incidence of stroke (RR, 1.39; 95% CI, 1.1-1.77) and a 12% increase in cardiovascular disease (RR, 1.12; 95% CI, 1.01-1.24).

    Study Design: RCTs, observational studies.
    Internal Validity: Good.
    Consistency: Good.
    External Validity: Poor.

    Interventions With Adequate Evidence of Decreased Risk of Breast Cancer

    Selective estrogen receptor modulators (SERMs)


    Based on solid evidence, tamoxifen and raloxifene reduce the incidence of breast cancer in postmenopausal women, and tamoxifen reduces the risk of breast cancer in high-risk premenopausal women. The effects observed for tamoxifen and raloxifene show persistence several years after active treatment is discontinued, with longer duration of effect noted for tamoxifen than for raloxifene.[4]


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