The presence of the hormone estrogen affects the growth of as many as two-thirds of all breast cancers, and the hormone progesterone similarly affects 40 percent to 50 percent of all breast cancers, according to the National Cancer Institute. In these cases if the level of hormones that reaches the cancer cells is decreased, then the cells may stop growing and die. Whether your cancer is affected by hormones can be determined by hormone receptor tests. When your tumor is removed, your doctor will perform estrogen and progesterone receptor tests on a tissue sample. These tests show whether the tumor has hormone receptors.
Hormone therapy seeks to prevent breast cancer cells from receiving stimulation from estrogen.
Tamoxifen is the most commonly used type of hormone therapy. Once a breast cancer cell accepts tamoxifen in the place of true estrogen, the cell is no longer able to receive the estrogen that it needs to divide. Eventually, it stops growing and dies. Tamoxifen is taken daily in pill form, usually for 5 years. Because the drug doesn't distinguish between cancer and normal cells, some normal cells will be affected.