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    All fractures were reduced by SERMs, primarily noted with raloxifene but not with tamoxifen. Reductions in vertebral fractures (34% reduction) and small reductions in nonvertebral fractures (7%) were noted.[4]

    Magnitude of Effect: 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 the prevention of noninvasive tumors. A meta-analysis showed an overall 38% risk reduction for SERMs in breast cancer incidence. With 5 years of treatment, 42 women would need to be treated to prevent one breast cancer in the first 10 years of follow-up.

    Study Design: RCTs.
    Internal Validity: Good.
    Consistency: Good.
    External Validity: Good.


    Based on solid evidence, tamoxifen treatment increases the risk of endometrial cancer, which was apparent in the first 5 years of follow-up but not beyond; thrombotic vascular events (i.e., pulmonary embolism, stroke, and deep venous thrombosis); and cataracts. Many of these risks are reduced after active treatment with tamoxifen is discontinued. Based on solid evidence, raloxifene also increases venous pulmonary embolism and deep venous thrombosis but not endometrial cancer.

    Magnitude of Effect: Meta-analysis showed RR of 2.4 (95% CI, 1.5–4.0) for endometrial cancer and 1.9 (95% CI, 1.4–2.6) for venous thromboembolic events. Meta-analysis showed the hazard ratio (HR) for endometrial cancer was 2.18 (95% CI, 1.39–3.42) for tamoxifen and 1.09 (95% CI, 0.74–1.62) for raloxifene. Overall, HR for venous thromboembolic events was 1.73 (95% CI, 1.47–2.05).

    Study Design: RCTs.
    Internal Validity: Good.
    Consistency: Good.
    External Validity: Good.

    Aromatase inhibitors or inactivators


    Based on solid evidence, aromatase inhibitors or inactivators (AIs) reduce the incidence of new breast cancers in postmenopausal women who have an increased risk.

    Magnitude of Effect: After a median follow-up of 35 months, women aged 35 years and older who had at least one risk factor (age >60 years, a Gail 5-year risk >1.66%, or ductal carcinoma in situ with mastectomy) and who took 25 mg of exemestane daily had a decreased risk of invasive breast cancer (HR, 0.35; 95% CI, 0.18–0.70). The absolute risk reduction was 21 cancers avoided out of 2,280 participants over 35 months. The number needed to treat was about 100.

    Study Design: One RCT.
    Internal Validity: Good.
    Consistency: One study in women with no history of breast cancer but consistent with RCTs in women with history of breast cancer.
    External Validity: Good for women who meet inclusion criteria.

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