A new study confirms previous research which shows that the drug tamoxifen Study Confirms Benefits of Tamoxifen But Also Identifies Potential Risk (dateline July 12, 2001) | Breast Health News | Imaginis - The Women's Health & Wellness Resource Network

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Study Confirms Benefits of Tamoxifen But Also Identifies Potential Risk (dateline July 12, 2001)

A new study confirms previous research which shows that the drug tamoxifen decreases the chances that breast cancer will return in patients who have previously been treated for the disease. However, the study also found that among patients who take tamoxifen and develop a new cancer in the opposite breast, that cancer may be more difficult to treat than if patients had not been using tamoxifen. While the results sound alarming, the number of women in the study that developed hard-to-treat cancers as a result of using tamoxifen were still small. Although the results provide experts with more information about tamoxifen, the researchers themselves emphasize that the study should not cause any woman to stop taking tamoxifen.

Tamoxifen is used to treat breast cancer and prevent the disease in women at high risk. Several studies have found that tamoxifen is effective at reducing tumor size, preventing a recurrence of breast cancer, and increasing patient survival time. This recent study on tamoxifen also found that tamoxifen lowers the risk that cancer will return in patients who have been treated for breast cancer. Because of this finding, the researchers do not advocate any changes to current clinical guidelines regarding the use of tamoxifen.

The study was conducted by Christopher I. Li, MD, MPH and his colleagues from the Fred Hutchinson Cancer Research Center in Seattle, Washington. Dr. Li’s team studied nearly 9,000 women who had been diagnosed with breast cancer between 1990 and 1998. Half of the women had used tamoxifen and the other half had not. Of the women who used tamoxifen, 89 developed a second cancerous tumor compared with 100 women who did not use tamoxifen. However, of the women who developed second tumors, the women on tamoxifen were more likely to develop estrogen-receptor negative tumors—a type that is usually more difficult to treat and associated with a lower survival rate (see explanation below).

Therefore, the researchers concluded that while tamoxifen reduces the chances that women who have had breast cancer will develop another cancer in the opposite breast, the small number of women who do get cancer again are more likely to have estrogen-receptor negative (ER-negative) cancers. The researchers are unsure why tamoxifen users tend to develop ER-negative cancers instead of estrogen-receptor positive (ER-positive) cancers. One theory is that tamoxifen prevents the growth of ER-positive cancer cells but does not stop the growth of ER-negative cancer cells, so in tumors that contain both types of cells, the ER-positive cells die but the ER-negative cells remain and develop into a new tumor.

ER-positive breast cancers are more common than ER-negative breast cancers. With ER-positive cancers, the cancer cells depend on the hormone estrogen for survival. Several "anti-estrogen" drugs have been created to cut off the body’s supply of estrogen to these cancer cells, thereby starving the cells. By contrast, ER-negative breast cancers do not need estrogen to survive and do not typically respond to anti-estrogen drugs. Physicians determine whether patients have ER-positive or ER-negative breast cancers by examining the breast tissue cells under a microscope after a breast biopsy or breast cancer surgery.

Estrogen-Receptive Positive (ER-Positive) Breast Cancer Estrogen-Receptive Negative (ER-negative) Breast Cancer
  • 66% of breast cancers are ER-positive.
  • Depend on estrogen for survival
  • Tend to respond to anti-estrogen drugs, such as tamoxifen, Arimidex, Aromasin, Fareston, etc.
  • 33% of breast cancers are ER-negative or hormone status is unknown.
  • Do not depend on estrogen for survival
  • Do not typically respond to current drug therapies


Dr. Li and his colleagues believe their study helps with the overall understanding of tamoxifen and underscores the need to develop drugs to treat ER-negative breast cancers. However, in an accompanying editorial in the Journal of the National Cancer Institute, Sandra Swain, MD, says that Dr. Li’s study is inconsistent with other data in medical literature and has other limitations that raise questions about its conclusions.

The bottom line is that the findings do not warrant any changes in current clinical guidelines. Because tamoxifen significantly reduces the chances that cancer will develop in the opposite breast, breast cancer patients who have been recommended to take tamoxifen should continue doing so.

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