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Breast Cancer Health Center

Tamoxifen for Breast Cancer Treatment and Prevention

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Tamoxifen is a medication that blocks the action of the female hormone estrogen in the body. For more than 30 years, doctors have prescribed tamoxifen to fight tumor growth in women who have breast cancer with tumors that are fueled by estrogen. Tamoxifen was the first selective estrogen receptor modulator (SERM) to be approved by the FDA for both prevention and treatment of breast cancer in high-risk women. 

What are SERMs?

SERMs are specific chemicals that act like estrogen in certain tissues but not others. For instance, they act like estrogen in the bones, uterus, and liver. Yet they don't act like estrogen in tissues such as the breast. SERMs, like tamoxifen, give some women the benefits of the hormone estrogen while helping them to avoid the serious risks of estrogen in the body.

How does tamoxifen work?

The exact mechanism of how tamoxifen works is still unknown. What is known is that some breast cancer cells are sensitive to the hormone estrogen. These cells have estrogen receptors and need estrogen to grow and spread. However, in order to grow and divide, estrogen must bind to the breast cancer cell receptors.

It's thought that tamoxifen, an anti-estrogen, binds and blocks these estrogen receptors on the surface of cells. That, in turn, prevents estrogen from binding to and activating the cell. Blocking estrogen from the estrogen-sensitive cancer cells stops the growth and reproduction of these deadly cells.

Why is tamoxifen prescribed?

Tamoxifen is prescribed for different reasons, including the following:

  • Treatment of advanced, early stage, and invasive breast cancer following surgery or radiation
  • Prevention and treatment of cancers that test positive for estrogen receptors (ER+)
  • Prevention and treatment of cancer in one breast to reduce the risk of cancer in the second breast
  • Prevention of invasive breast cancer in women at high risk
  • Treatment of ductal carcinoma in situ (DCIS) following surgery and radiation
  • Stimulation of ovulation
  • Use in other cancers such as ovarian cancer, uterine cancer, melanoma, and brain tumors

When tamoxifen is used in higher than normal doses, the medication may even kill breast cancer cells that are not estrogen sensitive.

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