Tamoxifen

How Does It Work?

Tamoxifen is part of a group of medications called selective estrogen receptor modulators (SERMs). SERMs have both estrogenic (estrogen-like) effects and anti-estrogenic effects. Tamoxifen has anti-estrogenic effects on the breasts, while it has estrogen-like effects on bones, helping to strengthen them.
 
Many breast cancers are sensitive to the estrogen hormone, meaning that the tumor grows with the help of estrogen. When a tumor is sensitive to estrogen, it has receptors on the outer surface of its cells. Estrogen fits into these receptors like a key opening a lock. When this connection is made, the cancer grows. Breast cancers that have estrogen (and/or progesterone) receptors are known as hormone receptor-positive tumors.
 
Tamoxifen binds to estrogen receptors in the breast, preventing estrogen from binding. By decreasing these effects of estrogen, the drug can help decrease the growth of these breast cancers. This type of breast cancer treatment is known as hormonal therapy (see Breast Cancer Hormone Treatment).
 

Effects

Several studies have looked at the effects of tamoxifen in women with breast cancer, especially when used as an adjuvant therapy of early breast cancer. Adjuvant therapy for breast cancer is treatment that follows breast cancer surgery (with or without radiation). It is given to help prevent the cancer from returning. These studies included women with cancer that was either hormone receptor-positive or hormone receptor-unknown. More women survived after 5 and 10 years on tamoxifen compared to those not taking the drug.
 
Other studies have shown that tamoxifen can help prevent breast cancer in women with ductal carcinoma in situ (DCIS) who have had surgery and radiation. About 17 in every 1,000 women not taking the drug developed invasive breast cancer per year, compared to only about 10 in every 1,000 women per year taking tamoxifen.
 
Tamoxifen can also prevent breast cancer in women who are at high risk for the disease. In studies, the drug reduced the risk of developing breast cancer by as much as 44 percent.
 
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