Tamoxifen can prevent breast cancer in high-risk women, although most studies have focused on the prevention of cancers during the active treatment phase. The International Breast Cancer Intervention Study I (IBIS-I) investigators now report that tamoxifen taken for 5 years continues to prevent breast cancers for up to 5 additional years after stopping treatment. Moreover, most side effects disappear once tamoxifen therapy is stopped.
The IBIS-I trial randomized 7,154 women at increased risk of breast cancer to receive either tamoxifen or placebo for 5 years. After an initial median follow-up of 49.6 months, tamoxifen reduced the risk of breast cancer by 32%. The investigators now find that, with a median follow-up of 8 years, tamoxifen continues to provide a significant reduction (27%) in the overall incidence of breast cancers, and this effect was observed for up to 10 years after randomization. The reduction was greatest for oestrogen receptor-positive tumours, which were 26% lower in the tamoxifen group during tamoxifen treatment and 44% lower in subsequent years.
