In a healthy state, certain processes occur, then the cell moves on to the next step, finally creating a new cell. At points during the cell cycle, everything stops at predetermined points so that checks can be made to ensure that abnormal cells don't replicate. Cancer cells have abnormal cell cycles because they have lost those checkpoints. Cancer cells proceed through cell division at break-neck speed. One of the ways traditional chemotherapy works is to restore a checkpoint and stop the cell cycle.
Tamoxifen is used as a chemotherapeutic agent because it stops the cell cycle.
One area that might be damaged is tumor-suppressor gene p53. Gene p53 is a player in the process that stops the cell cycle and makes sure cancer cells don't replicate. In a healthy person, p53 sends signals that stop the cell cycle when abnormal cells are involved and cause them to self-destruct. Using human breast cancer cells, researchers in France showed that tamoxifen stops p53 from working. While this may sensitize cancer cells to the effects of chemotherapy, the same phenomenon in a healthy person would cripple p53's ability to stop cancer.