Poverty and higher out-of-pocket drug costs may be the main reason that some women don't stick with hormone therapy designed to prevent breast cancer from coming back, a recent U.S. study suggests.

Researchers analyzed data on about 8,700 women who had breast cancer and at least one prescription for pills to curb production of the hormone estrogen - which can fuel tumor growth - or pills to stop estrogen from attaching to cancer cells.

Overall, slightly more than one-third of these women failed to stick with their prescribed hormone therapy regimens.

While previous research has found race and ethnicity may influence whether women take their hormones, the current study found that most of the differences in adherence between minorities and white patients could be explained by drug costs, said lead author Albert Farias of the University of Texas Health Science Center at Houston.

"The cost of the medications matters," Farias said by email.

"This finding is highly important because it adds to the evidence that little to no-cost prescription drugs may be an effective strategy to improve endocrine therapy adherence, which could reduce the risk of breast cancer recurrence and mortality," Farias added.

To see what factors may influence whether women take hormone therapy, researchers looked at data on women aged 65 and older enrolled in Medicare Part D prescription drug plans from 2007 to 2009.

Overall, about 63 percent of the women consistently filled prescriptions for hormone therapy throughout the entire 12-month study period, researchers report in the Journal of Clinical Oncology.

More on this...

To see how money influenced medication use, researchers examined women's total out-of-pocket costs including copayments, deductibles and co-insurance.

They divided average monthly costs into quintiles, with the average costs in the lowest group totaling no more than $2.65 and typical monthly costs in the highest group exceeding $105.55.

When women had average out-of-pocket costs of more than $10, they were significantly less likely to stick with hormone therapy than patients with the lowest monthly outlays, the study found.

In addition, unmarried women and patients living in poor communities were less likely to remain on hormone therapy than patients with husbands or in affluent neighborhoods.

Once researchers accounted for costs and other factors that can influence drug adherence, they didn't find meaningful differences between white, black and Hispanic women.

Asian women, though, were about 48 percent more likely to stick with their drugs than white patients.

In addition, black women were much less likely to stick with one drug - tamoxifen - than white women. This drug can carry a higher price tag than other hormone therapies known as aromatase inhibitors.

One limitation of the study is that it only included women 65 and older, so the results might not reflect what would happen with younger patients, the authors note. The study also relied on prescription records to assess medication compliance, which doesn't always reflect whether women take their drugs.

This year, Medicare may cover a larger chunk of women's out-of-pocket costs for breast cancer drugs because of subsidies under the Affordable Care Act, the authors also note.

Even so, the findings highlight how costs may influence outcomes for women with breast cancer, said Dr. Alana Biggers, a researcher at the University of Illinois- Chicago College of Medicine who wasn't involved in the study.

"Black women and Hispanic women were more likely to reside in areas with a lower per-capita income and in areas with less educational attainment compared with white women," Biggers said by email. "We can imply from this that the white women enrolled in the study had more financial resources to purchase medications and continue treatment longer."

Many patients simply don't have the disposable income to spend on medications, noted Dr. Sharon Giordano, a researcher at the University of Texas MD Anderson Cancer Center who wasn't involved in the study.

"When medications are more expensive, fewer patients will take their prescribed treatments, and patients will be at higher risk of recurrence and death from breast cancer," Giordano said by email. "This study highlights the importance of controlling medication costs."