Hormone therapies are designed to inhibit the growth of cancer cells by blocking the hormones they need in order to grow. Estrogen and progesterone are two of these hormones, produced naturally by a woman’s body. When these hormones attach to special proteins on cancer cells called hormone receptors, the malignant cells containing these receptors are stimulated to grow.

The key with hormone therapies is that they only work on cancer cells that have receptors for these hormones. As a result, a woman is only eligible for hormone therapy if her tumor is either estrogen receptor positive (ER positive) or progesterone receptor positive (PR positive). The receptor status of a tumor is determined by testing tissue that is removed during a biopsy.

Some hormone therapies, like the drug tamoxifen, block the action of the hormone by attaching to the receptor on the surface of the cancer cell. Tamoxifen is used to treat certain types of breast cancer and to prevent breast cancer in women who have had ductal carcinoma in situ.

Other therapies, like aromatase inhibitors, lower the level of estrogen in the body, so that the cancer cells cannot get the estrogen they need to grow. Hormone therapies are used for early, locally advanced, and metastatic breast cancer.