After weighing the risk of serious side effects with the benefits of a breast cancer prevention drug, a study found that the drug’s benefits outweighed risks for most, but not all, women. These findings were published in the Journal of the National Cancer Institute (2014; doi:10.1093/jnci/dju354).

This information 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 lead author Hazel Nichols, PhD, an assistant professor of epidemiology in the University of North Carolina Gillings School of Public Health in Chapel Hill.

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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 US 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 years 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.

One of the biggest takeaways from Nichols’ analysis is that it shows that there are tools that can help women and their doctors determine whether preventive drugs such as 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 into account.

They found that the drug’s benefits outweighed the risks for 74% of the women included in the analysis, but not for another 20%, or 1 in 5 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% less likely than white women to see their benefits exceed risks associated with taking the drug.

The study also found that many women discontinued the drug earlier than recommended. Of the 788 tamoxifen users in the study, 46% stopped taking the drug before the recommended 5 years. While the study looked at several factors such as age, race, and breast cancer risk, Nichols said none of those were strong predictors of who decided to stop taking tamoxifen early.