Some breast cancer cells are estrogen sensitive. In other words, estrogen binds to these cells and stimulates them to grow and divide.
Tamoxifen prevents the binding of estrogen from occurring.
This stops the cell from growing and in doing so, prevents or delays recurrence. While tamoxifen blocks the effect of estrogen on breast tissue, it acts like a weak estrogen in other body systems. This means that women who take tamoxifen may share some of the beneficial effects of taking estrogen replacement therapy, such as a decreased risk of osteoporosis and a decreased risk of heart disease. Current thinking is that tamoxifen blocks the action of estrogen only as long as it is taken. Recent research comparing five years of adjuvant tamoxifen therapy with more than five years has indicated that no additional benefit is obtained from extending the duration of tamoxifen therapy past five years.