More cases occur in a study involving women at increased risk. This difference in case numbers between the groups is very significant. For the larger of the European studies, the numbers of cases were 36 and 34, respectively. For the smaller of the two studies, the numbers of cases were 22 in the placebo group and 19 in the tamoxifen group. The larger number of cases in the BCPT provides more confidence that the observed differences are real.
Tamoxifen can interact with the same receptor on cells that binds estrogen.
Consequently, there is a theoretical possibility that estrogen use may block the effect of tamoxifen. In the BCPT, the use of estrogen was not permitted so that the effect of tamoxifen alone could be evaluated. An integral part of the BCPT was an independent Endpoint Review, Safety Monitoring, and Advisory Committee. It was concluded that the value of additional information that might be gained from continuing the study did not outweigh the benefits of making tamoxifen available to the participants in the placebo group and other women at increased risk of breast cancer. The NCI's confidence in the results of the BCPT showing that tamoxifen can reduce breast cancer in women at increased risk remains firm.