Tamoxifen unsuitable for primary prevention of breast cancer: uncertain efficacy, clear risks

(1) Three large placebo-controlled trials conducted in the United States, the United Kingdom and Italy have tested tamoxifen (20 mg/day) in primary prevention of breast cancer. The British and American trials involved women at particularly high risk. (2) Only the American trial concluded that tamoxifen was effective. The results suggested that treating 130 women with tamoxifen for 5 years would avoid two cases of invasive breast cancer, but at a cost of one severe adverse event (either deep vein thrombosis/pulmonary embolism or uterine cancer). No impact on mortality was documented. (3) No satisfactory reason has yet been forwarded for the conflicting results of the American and British trials. The negative results of the Italian trial are difficult to interpret because of inadequate statistical power. (4) As the preventive effect of tamoxifen is at best limited, and because of uncertainties on the consequences of long-term treatment, prescribing tamoxifen for primary prevention of breast cancer is not justified. (5) Tamoxifen currently has no place in the primary prevention of breast cancer, except in the clinical trial setting.