Estrogen is an essential female hormone that is produced by the ovaries and adrenal glands.  It serves many critical functions in the body, including developing the female sex organs in puberty, preparing the breasts and uterus for pregnancy in adulthood, and maintaining cardiovascular and bone health. Without estrogen, the female body is unable to sustain pregnancy and is susceptible to heart disease and osteoporosis.

Estrogen can also cause some cancers to grow. The breasts, uterus and other female organs are composed of cells that have estrogen receptors and are stimulated to grow when exposed to estrogen.  Estrogen circulating in the blood binds to the estrigeb receptors and stimulates growth-related activities in the cell. When cells that have estrogen receptors become cancerous, exposure to estrogen increases the cancer's growth. Cancer cells that have estrogen receptors are referred to as estrogen receptor-positive (ER-positive) cancers.

The growth of ER-positive breast cancer cells can be prevented or slowed by reducing the exposure to estrogen. This is the goal of hormonal therapy for breast cancer. However, a reduction in estrogen levels can also result in side effects because estrogen is necessary for important body functions, such as bone growth and cardiovascular health. Lower estrogen levels lead to decreased bone density and heart disease.

Tamoxifen is an anti-estrogen drug that has historically been a mainstay of hormonal therapy. However, several newer hormonal therapy drugs, referred to as anti-aromatase agents, have proven to be superior to tamoxifen for the treatment of patients with ER-positive breast cancer and are associated with fewer side effects. Furthermore, a new class of drugs called estrogen-receptor antagonists work in a similar way as tamoxifen and have been shown to benefit some patients with breast cancer.