The majority (75%) of breast cancers express hormone receptors for estrogen (ER) and progesterone (PR).  These receptors can use estrogen and progesterone to help the cancer cells grow so that drugs that can block or inhibit the function of the hormone or hormone receptors are used in treatment. 

The targeted anti-estrogen medications are the mainstay of treatment for those patients with hormone receptor positive breast cancers. The type of anti-estrogen therapy depends on menopausal status.

Premenopausal Women: The majority of premenopausal women will take the selective estrogen receptor modulator (blocks estrogen receptors in the breast) Tamoxifen. The drug is an oral pill that competes with estrogen for its receptor and inhibits or retards the growth of the breast cancer cells. The medication is generally taken once a day as a 20 mg tablet. Generally it is taken daily for 5 years but new evidence suggests especially high risk women may benefit from taking the medication for 10 years. Some premenopausal women may start Tamoxifen and if become menopausal switch to one of the aromatase inhibitor medications listed below. Side effects are generally hot flashes, blood clots, and in some patients the medication can increase the incidence of uterine cancer. There is some incidence of depression and loss of sex drive. Cataracts and stroke can be increased as well. Women need to be monitored while taking Tamoxifen. Tamoxifen is also used for prevention in high risk patients to reduce breast cancer risk for those with atypical cells on biopsy including ADH, ALH, LCIS and may be used to reduce risk in patients that are BRCA mutation carriers or women with family history of breast cancer. Tamoxifen is also indicated for women with non-invasive ductal carcinoma in situ to reduce recurrence in the same breast and reduce breast cancer from developing in the opposite breast.

Postmenopausal Women: The majority of these patients take an anti-estrogen medication called aromatase inhibitors or AIs. There are three of these medications: anastrozole (Arimidex), exemestane (Aromasin), and letrozole (Femara). Following menopause the ovaries cease making estrogen and most of the estrogens are produced in fat cells. These medications prevent the fat cells from making estrogen and hence the tumor cells are starved from estrogen and die. These medications can decrease the risk of breast cancer recurrence by about 50%. These medications are generally not given to premenopausal women because their ovaries are still making lots of estrogen and the medications do not block that estrogen production. The medications are all taken as daily pills generally for 5 years. Side effects may include bone loss, joint aches, hot flashes, and even some complaints of cognitive changes. Bone density is generally followed by an oncologist using a dexa scan.

Other Anti-Estrogen Medications Used in Breast Cancer: 

Luteinizing hormone-releasing hormone- (LHRH) Leuprolide (Lupron), goserelin (Zoladex), triptorelin (Trelstar) are a class of medications that can shut down the ovaries from making estrogen. They are sometimes used in early premenopausal breast cancer patients or those with breast cancer that has spread or becomes resistant to Tamoxifen. The medications are injected monthly and may cause hot flashes, mood swings, loss of libido, and osteoporosis. Some oncologists may use these medications in combination with aromatase inhibitors in high risk premenopausal patients that cannot tolerate or have contraindication to Tamoxifen. 

Fulvestrant- (Faslodex) Fulvestrant is used in hormone refractory recurrent or breast cancer that has spread which is estrogen positive. The medication decreases estrogen receptors on the cancer cell surface. It is generally given as an injection.