Tamoxifen for breast cancer prevention

11 December 2014


This UK coordinated breast cancer prevention study investigated whether the breast cancer drug tamoxifen is effective in reducing the incidence of breast cancer in women at high risk for breast cancer (mostly due to a strong family history).

Scientific Title

ANZ 92P1: International Breast cancer Intervention Study (IBIS)-I


The belief that oestrogen is the primary promotional factor for breast cancer has a long history and is now well established.  Early trials of the drug tamoxifen when used to treat early breast cancer showed a striking reduction (50%) of new tumours in the opposite breast. With its apparently low side effect profile, it was proposed that tamoxifen prevention might be a suitable approach to reducing breast cancer in high-risk women.

The first tamoxifen prevention ‘pilot’ trial commenced at the Royal Marsden Hospital (England) in 1986 and then evolved into a full-scale international prevention study.


The results of the IBIS-I study found that tamoxifen can prevent breast cancer in healthy women at increased risk, with 32% fewer breast cancers diagnosed in IBIS-I women who took tamoxifen. Additional long-term follow up showed that the preventative effect of tamoxifen continued for at least 20 years, so it continued for at least 15 years after completion of the 5 years of tamoxifen therapy.

Main publication: Tamoxifen for prevention of breast cancer: extended long-term follow-up of the IBIS-I breast cancer prevention trial. Cuzick J, Sestak I, Cawthorn S, Hamed H, Holli K, Howell A, Forbes JF, on behalf of the IBIS-I Investigators. Lancet Oncology 2015; 16:67-75.

Quality of life sub-study

729 IBIS-1 participants randomised to tamoxifen or placebo completed health-related quality of life and global health status questionnaires at baseline, year 1, year 3, and year 5 on the study. Conclusions from this study showed that tamoxifen does worsen vasomotor (e.g. hot flushes) and gynecological symptoms in an extra 10-12% of women over placebo. These effects do not appear to adversely affect overall health status, so they are unlikely to outweigh the benefits of reducing breast cancer incidence.