Hormone therapy is sometimes used after surgery to decrease the change of the cancer returning or after it has already spread to shrink and contain it. This therapy is recommended for women who have a type of breast cancer that is sensitive to hormones—estrogen receptor positive (ER positive) and progesterone receptor positive (PR positive) cancers. There are three primary ways to block hormones:

Medicines that block hormones from attaching to cells:
Tamoxifen is the most commonly used drug to block estrogen from attaching to an estrogen receptor in cancer cells, which slows the growth and can kill cancer cells. This drug can be used in both pre- and post-menopausal women.

Possible Side Effects

Medicines that stop the production of estrogen after menopause
such as a group of drugs called aromatase inhibitors that block the action of an enzyme that converts androgens in the body to estrogen.

Possible Side Effects

Medicines or surgery that stops hormone production in the ovaries for premenopausal women. Additionally, some women can have surgery in the form of prophylactic oophorectomy to surgically stop hormone production.