The International Breast Cancer Intervention Study I (IBIS-I) was designed to investigate the use of tamoxifen in preventing breast cancer in women with a higher risk of developing the disease. Recruitment of women to IBIS-I ended in March 2001 and it recruited 7154 women from 36 centres in 9 countries. 

The results of the study showed that tamoxifen reduced the incidence of breast cancer by one third in these high risk women but with some serious side effects. IBIS-II was designed to continue the work started in IBIS-I by examining the role of anastrozole in the prevention of breast cancer which we hope will reduce breast cancer by even more than tamoxifen with less serious side effects.