Hormonal therapy is a medicine that reduces estrogen or blocks its effects to slow down cancer cell growth in the body. Estrogen is a hormone that stimulates the growth of cancer cells. If this is reduced or its effects are blocked, then cancer cell growth is likewise reduced. That is why hormonal therapy is also called anti-estrogen therapy.
It is important to note that this treatment is applicable only if the breast cancer is hormone-receptive-positive. Tamoxifen is the popular choice for most women, but if one is already post-menopausal, doctors would advise aromatase inhibitors such as Arimidex, Aromasin and Femara.