Three clinical trials on the use of tamoxifen to prevent breast cancer have reported mixed results. The overall evidence supports a reduction in the risk of breast cancer, but whether this benefit outweighs the risks and side-effects associated with tamoxifen is unclear.

We undertook a double-blind placebo-controlled randomised trial of tamoxifen, 20 mg/day for 5 years, in 7152 women aged 35-70 years, who were at increased risk of breast cancer. The primary outcome measure was the frequency of breast cancer (including ductal carcinoma in situ). Analyses were by intention to treat after exclusion of 13 women found to have breast cancer at baseline mammography.

After median follow-up of 50 months (IQR 32-67), 69 breast cancers had been diagnosed in 3578 women in the tamoxifen group and 101 in 3566 in the placebo group (risk reduction 32% [95% CI 8-50]; p=0.013). Age, degree of risk, and use of hormone-replacement therapy did not affect the reduction. Endometrial cancer was non-significantly increased (11 vs 5; p=0.2) and thromboembolic events were significantly increased with tamoxifen (43 vs 17; odds ratio 2.5 [1.5-4.4], p=0.001), particularly after surgery. There was a significant excess of deaths from all causes in the tamoxifen group (25 vs 11, p=0.028).

Prophylactic tamoxifen reduces the risk of breast cancer by about a third. Temporary cessation of tamoxifen should be considered and the use of appropriate antithrombotic measures is recommended during and after major surgery or periods of immobilisation. Prophylactic use of tamoxifen is contraindicated in women at high risk of thromboembolic disease. The combined evidence indicates that mortality from non-breast-cancer causes is not increased by tamoxifen. The overall risk to benefit ratio for the use of tamoxifen in prevention is still unclear, and continued follow-up of the current trials is essential.