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After weighing the risk of serious side effects with the benefits of a breast cancer prevention drug, a study led by a University of North Carolina at Chapel Hill researcher found that the drug’s benefits outweighed risks for most, but not all women.

Hazel Nichols, PhD

The study’s findings may help women and their doctors make decisions about who may get the most benefit out of taking the drug tamoxifen, which has been shown to have been adopted by only a slim margin of women eligible to take it.

“It’s important because it highlights that the estimated benefit is not the same for all women, and so women need to go and have a very specific conversation with their providers about what their health looks like at that time, and whether this is a reasonable option for them,” said Hazel Nichols, PhD, an assistant professor of epidemiology in the University of North Carolina Gillings School of Public Health.

Nichols was the lead author of the study, which was published online in the Journal of the National Cancer Institute in December, and in print in January.

Part of the drive for doing the analysis, Nichols said, is the fact that few women eligible to take tamoxifen for breast cancer prevention are actually choosing to take it, although it’s been shown to reduce invasive breast cancer risk. A panel of U.S. health experts has encouraged doctors to offer chemoprevention drugs to women at higher risk for breast cancer and at low risk for side effects. Tamoxifen was first approved in 1998 for women older than 35 with those characteristics.

“Tamoxifen is very good at preventing breast cancer, but it also has to be weighed against an increased risk of uterine cancer, stroke or blood clotting negative effects,” Nichols said.

Nichols said that one of the biggest takeaways from the analysis she led is that it shows that there are tools that can help women and their doctors determine whether preventive drugs like tamoxifen are right for them.

Using a mathematical tool developed at the National Cancer Institute, the researchers were able to weigh the risks and benefits of taking the drug for a group of 788 women who were taking tamoxifen and did not have a history of breast cancer. The analysis took the women’s age, race, breast cancer risk, and risk for serious side effects such as stroke, uterine cancer, and cataracts from taking tamoxifen into account.

They found that the drug’s benefits outweighed the risks for 74 percent of the women included in the analysis, but not for another 20 percent, or one in five women. Women who had hysterectomies before starting the drug were more 11 times more likely to have a favorable risk-to-benefit profile. Younger women were also more likely to benefit, but African-American women were 65 percent less likely than white women to see their benefits exceed risks associated with taking the drug.

The study also found that many women stopped taking the drug before the recommended five years were up. Out of the788 tamoxifen users in the study, 46 percent stopped taking the drug before the recommended five years, the study found. While the study looked at several factors such as age, race and breast cancer risk, she said none of those were strong predictors of who decided to stop taking tamoxifen early.

Dr. Nichols said the study’s finding that the benefits did not outweigh the risks for one in five women is a “reminder” that factors like a woman’s age and her hysterectomy status should be considered when making decisions about chemoprevention.

“I think we have seen that not all women who take tamoxifen for chemoprevention will have equivalent evidence that the risks outweigh the benefits, but the risk-benefit tool is easy to apply to estimate whether a woman’s benefits are likely to outweigh the risks,” she said.

For the study, the researchers drew upon The Sister Study, a National Institute of Environmental Health Sciences-led study that explored risk factors for breast cancer. More than 50,000 U.S. and Puerto Rican women aged 35 to 74 years joined the study. Women were eligible if they were breast cancer-free, but had a sister diagnosed with breast cancer. The Sister Study gave the researchers an opportunity to evaluate a large group of women at higher risk for breast cancer outside of a clinical trial, Nichols said.

The UNC-Chapel Hill-led study was supported by the Intramural Research Program of the National Institutes of Health, the National Institute of Environmental Health Sciences and the National Center for Advancing Translational Sciences.

Study co-authors included Lisa A. DeRoo of the Department of Global Public Health and Primary Care at the University of Bergen in Norway; Daniel R. Scharf of Westat; and Dale P. Sandler of the National Institute of Environmental Health Sciences Epidemiology Branch.