Tamoxifen citrate has been the most commonly prescribed drug to treat breast cancer, since the approval of the drug by the U.S. Food and Drug Administration (FDA) in the 1970s. Tamoxifen is taken orally in pill form. The brand name of tamoxifen citrate is Nolvadex.
Tamoxifen citrate is an example of a type of drug called a selective estrogen receptor modulator (SERM). Tamoxifen works by binding to estrogen receptors on estrogen-dependent breast cancer cells, thereby competitively blocking their ability to absorb estrogen, preventing their ability to "turn on" estrogen-sensitive signals in DNA, suppressing DNA synthesis of the cancer cells, and blocking multiplication of the cancer cells. Without having bound estrogen to stimulate their growth, the breast cancer cells “starve” and die.
Also, tamoxifen blocks the production of insulin-like growth factor-1 (IGF-1), a molecule that otherwise could help cancer cells multiply. Furthermore, tamoxifen stimulates the synthesis of transforming growth factor B (TGF-b), a molecule that suppresses the multiplication of cancer cells.
Tamoxifen reduces the risk of recurrence and metastasis of estrogen receptor-positive (ER-positive) breast cancer. Use of tamoxifen for 5 years in post-menopausal women who had been treated for ER-positive breast cancer resulted in an 81.4% rate of disease-free survival (i.e., survival with no evidence of recurrence of disease).
If a woman who has had ER-positive breast cancer has taken tamoxifen for 5 years, however, it generally is recommended that she consider taking an aromatase inhibitor.
In 1998, Tamoxifen became the first drug to be approved by the FDA for the prevention of breast cancer. Research showed that tamoxifen reduced the chances of developing breast cancer by 50% in women at high risk for the disease.
As tamoxifen blocks the action of estrogen, some side effects are possible.