WASHINGTON — Using tamoxifen in hopes of preventing breast cancer would benefit about 2.4 million women despite the drug's serious side effects, federal researchers reported Tuesday.
Tamoxifen is a breast cancer treatment that also can cut by almost half a still-healthy woman's risk of developing the disease. Deciding to use it preventively is a difficult choice, however, because tamoxifen can cause uterine cancer and potentially fatal blood clots.
The new study in the Journal of the National Cancer Institute marks the first national estimate of how many of those women could benefit from tamoxifen -- and who are optimal candidates.
"Chemoprevention of breast cancer with tamoxifen is a choice, and it's not appropriate for all women," said study author Andrew Freedman, an epidemiologist at the cancer institute. "We would like to have those women who would likely benefit from tamoxifen taking the drug."
About 600,000 American women take tamoxifen each year, most to treat cancer, not prevent it, says leading manufacturer AstraZeneca.
Freedman and colleagues estimated that 10 million women ages 35 to 79 are eligible for tamoxifen because they are at increased risk of developing breast cancer within five years, as assessed by such factors as family cancer history.
But subtract those most at risk of serious side effects, and Freedman estimated 2.47 million women could benefit from tamoxifen without suffering undue harm. Not all of them will get breast cancer because doctors can't predict that precisely.
Most potential tamoxifen beneficiaries are white; only 43,000 are African American.
Although black women are more likely to die than whites if they get breast cancer, the disease is far less common among black women, Freedman said. He did not have enough data to calculate tamoxifen's effects in other racial and ethnic groups.