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Prevention and Screening

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Use of tamoxifen to prevent breast cancer should be weighed against an individual woman's potential benefits and risks

Physicians considering use of tamoxifen to reduce a woman's risk of developing breast cancer or recurrence of breast cancer should weigh her breast cancer risk against her risk for developing tamoxifen-related vascular problems or other cancers, according to a study that was supported in part by the Agency for Healthcare Research and Quality (HS09796). Tamoxifen reduces the risk of breast cancer by acting against the effects of the hormone estrogen in breast tissue, but it also acts like estrogen in other tissues, which can create other problems.

This meta-analysis of 32 clinical trials of women averaging 55 years of age and on tamoxifen for 4.3 years showed that tamoxifen was associated with a significantly increased risk of endometrial cancer, gastrointestinal cancers, strokes, and pulmonary emboli. On the other hand, tamoxifen use significantly decreased heart attack deaths and was associated with an insignificant decrease in heart attack incidence. Postmenopausal women taking tamoxifen had greater increases in risk for cancer than other women.

Although some of the increased risks associated with tamoxifen use seem large, the absolute risk for any of the problems studied occurring after 5 years of tamoxifen treatment is only 0.84 percent, explains Joseph Lau, M.D. of Tufts-New England Medical Center, the study's principal investigator. This corresponds to one adverse outcome for every 118 patients treated. In comparison, 159 women would have to be treated to prevent one occurrence of breast cancer in a woman with the minimum risk for which tamoxifen is indicated (1.66 percent risk of breast cancer after 5 years), assuming a risk reduction of 38 percent. For higher risk women (5 percent 5-year risk of breast cancer), 53 women would have to be treated with tamoxifen to prevent one occurrence of breast cancer.

More details are in "Meta-analysis of vascular and neoplastic events associated with tamoxifen," by R. Scott Braithwaite, M.D., Rowan T.. Chlebowski, M.D., Ph.D., Dr. Lau, and others, in the November 2003 Journal of General Internal Medicine 19, pp. 937-947.

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