In women who have breast cancer, proliferation of the breast cancer cells is often driven by estrogen, just as in the case of normal breast cells.

Since tamoxifen can block the effects of estrogen on breast cells, scientists predicted that breast cancer could be treated by using tamoxifen to interfere with estrogen-induced cell proliferation. Based on encouraging results obtained in experimental trials, tamoxifen was first approved for such use in breast cancer treatment in the 1970s.

The first step in treating women with breast cancer is to surgically remove the cancer from the breast. It is difficult to be certain that every cancer cell has been removed at the time of surgery because some breast cancer cells could have spread to surrounding tissues or other organs prior to the operation. Therefore, women often receive some type of treatment after surgery (adjuvant therapy) to prevent the growth of any cancer cells that might remain in the body. Studies show that when tamoxifen is used for this purpose, the risk of cancer recurrence is reduced.