Tamoxifen prevents breast cancers--at a price. (Study of More than 7,000 Women).

New study results confirm that treatment with tamoxifen is an effective but flawed way to prevent women from developing breast cancer.

For the time being, experts recommend prescribing tamoxifen to a well-defined, relatively small fraction of women: those who have a high risk of breast cancer and a low risk of thromboembolic disease.

The results from the International Breast Cancer Intervention Study (IBIS-I), published in September, documented that 5 years of tamoxifen treatment cut the risk of breast cancer by 32%, compared with placebo in a study that involved more than 7,000 women (Lancet 360[9336]:817-24, 2002).

Results from the multicenter trial also showed that women treated with tamoxifen had a 2.5-fold increased risk of thromboembolic events. IBIS-I is the fourth reported, randomized study to look at the effects of tamoxifen on breast cancer prevention. Two of the smaller, prior studies failed to show a benefit from tamoxifen, but researchers from the largest study, the U.S.-based National Surgical Adjuvant Breast and Bowel Project, found a 49% reduction in the risk of breast cancer when women were treated for 5 years.

The NSABP's entry criteria, a 5-year risk of breast cancer of at least 1.66%, has become the commonly accepted threshold for deciding whether a woman is at high enough risk to warrant receiving tamoxifen, Dr. Anne Blackwood-Chirchir, a medical epidemiologist at the University of Pennsylvania in Philadelphia, said in an interview. This threshold was recently endorsed by both the American Society of Clinical Oncology and the U.S. Preventive Services Task Force.

Factors used to predict a woman's baseline risk of cancer and her eligibility for tamoxifen include her age, race, family history of breast cancer, and a history of atypical hyperplasia on prior breast biopsies. These and other ...