“…there was a clear need to get an overall picture of tamoxifen's activity as a preventive agent.”

There is a lot of interest in the prevention of breast cancer with tamoxifen and early reports from three of the trials gave discordant results. This paper updated the follow-up of the trials and provided a consensus estimate of the risk reduction as well as estimates for the magnitude of the risk for the two major side effects—endometrial cancer and venous thromboembolic events.

The major significance was to provide firm estimates for the risk and benefits of tamoxifen as a preventive agent and to establish without doubt that it could prevent about one-half of estrogen receptor-positive breast cancer. The lack of effect on receptor-negative cancer was also clearly demonstrated.  It also reconciled different results from different trials and put the potential value of raloxifene into context. The data set was also large enough to separately examine results for women under and over 50 years of age.

This overview was a natural outgrowth from our report on the IBIS-I prevention trial. At that time other trials were showing discordant results and there was a clear need to get an overall picture of tamoxifen's activity as a preventive agent. European investigators had worked closely in the design of their studies, so it was not too difficult to pool the results.