Tamoxifen (Nolvadex®) has been used for more than 20 years to treat women with advanced breast cancer . It is a powerful drug that can reduce the risk of cancer recurring in the affected breast or developing in the other breast. The Food and Drug Administration (FDA) has more recently approved it for prevention in women who do not have breast cancer but are at high risk (this use is called chemoprevention). The FDA's approval was based on encouraging findings from the Breast Cancer Prevention Trial—tamoxifen decreased the incidence of invasive breast cancer in women at high risk by 49%.

Tamoxifen also increases the risk of several serious and possibly life-threatening medical conditions including endometrial cancer , pulmonary embolism , stroke , deep vein thrombosis , and cataracts . For these reasons, the FDA approved tamoxifen only for women age 35 and older with a five-year breast cancer risk of 1.67% or greater.

In the present study, published in the April 2, 2003 issue of the Journal of the National Cancer Institute , a group of researchers sought to quantify the number of women with a positive benefit/risk index—those who will experience benefits with minimal side effects from tamoxifen chemoprevention.

About the Study

A group of researchers from the National Cancer Institute analyzed data collected through the year 2000 National Health Interview Survey (NHIS), which is a questionnaire routinely administered to a sample of US households. This version of the NHIS included questions about breast cancer risk factors and screening practices; the responses were used to calculate a woman's absolute risk of developing breast cancer over the next five years.

The researchers determined which of the women who responded to the survey would be eligible to take tamoxifen as chemoprevention, based on the FDA guidelines (age 35 and older and a five-year breast cancer risk of 1.67% or greater). They also determined who would be eligible if the five-year breast cancer risk cutoff was slightly lower—1.5%.

Next, the researchers used a benefit/risk index to calculate the number of eligible women for which the benefits of tamoxifen—a reduction in breast cancer risk—would outweigh its risks of serious adverse effects.

The Findings

Based on the FDA criteria, 15.5% of all women in the U.S. aged 35-79 would be eligible to take tamoxifen to reduce their risk of breast cancer. The researchers further broke this number down by age group and race. Eligibility rates increased with age (as does risk of breast cancer), with 70-79 year-old women showing the greatest eligibility level (47.7%). More white women were eligible (18.7%) than black (5.7%) and Hispanic (2.9%) women; black and Hispanic woman are believed to be at lower risk of breast cancer than white women.

For the rest of their calculations, the researchers expanded the group of eligible women to include those with a five-year risk of invasive breast cancer of 1.5% (a lower number than the 1.67% defined by the FDA).

Among those who are eligible for tamoxifen chemoprevention, further calculation showed that just 4.9% of white women and 0.6% of black women would have positive benefit/risk index. The greatest benefit was among white women in the age groups 40-49 (8.1%) and 50-59 (8.5%). This seemingly small percentage of 4.9 translates into close to 2.5 million women. The researchers estimate that over the next five years, 58,148 invasive breast cancers will occur in these 2.5 million women. However, if each one of them takes preventive tamoxifen, 28,492 of these breast cancers—or 49%—could be prevented.

A very small benefit was shown for black women. This is because black women are at lower risk for breast cancer than white women, yet are at higher risk for several of the serious side effects of tamoxifen.

How Does This Affect You?

Most women will take any steps they can to reduce their risk of breast cancer. Tamoxifen appears to be a viable option for managing risk for some women, but it is also a powerful drug with significant side effects. The results of this study may help women and clinicians determine which groups of women will receive the most benefit from tamoxifen. However, these findings are estimates of the effects of tamoxifen across the entire population; several factors—both protective and damaging—were not included in the calculation. Therefore, these findings should be one of many factors considered when deciding if tamoxifen is right for you.

To determine the best methods for reducing your risk for breast cancer, you and your health care provider will review your medical history, present state of health, lifestyle, and preferences in medical care. Other methods that may help reduce the risk of breast cancer include not smoking , being physically active , maintaining a healthful weight , and limiting alcohol consumption. And while regular mammograms will not prevent breast cancer, they have been shown to reduce the risk of dying from it, particularly in women at high risk.