(USA TODAY) -- Half of breast cancer patients stop taking key medications ahead of schedule, a decision that can increase their risk of relapse and death, a new study shows.

The finding underscores the importance of working with patients to reduce side effects, especially at a time when patients are increasingly responsible for their own care, says Jennifer Obel, an American Society of Clinical Oncology breast cancer expert. Fifty cancer drugs are now available as pills that can be taken at home.

The study focused on women whose breast tumors are fueled by estrogen. In addition to surgery and radiation, many of these women typically are told to take hormonal therapies -- such as tamoxifen or an aromatase inhibitor -- for five years. These drugs, which prevent estrogen from stimulating tumor growth, can reduce the risk of relapse by 40% and cut the risk of death by 10%, says Patricia Ganz of the University of California-Los Angeles.

In a study of 8,769 women, however, only 49% of women took hormonal therapy pills for at least 4 1/2 years, according to the paper published today in the Journal of Clinical Oncology.

Authors say they can understand why women stop filling prescriptions. "Five years is a long time," says study author Dawn Hershman of Columbia University Medical Center. "As you move further out from your diagnosis, it becomes a distant memory. Some women don't want to be reminded."

Side effects also can lead people to stop filling prescriptions. About 40% of women taking aromatase inhibitors develop joint pain, Hershman says.

For some women, side effects of tamoxifen -- hot flashes, sleep problems, mood changes, loss of libido and vaginal dryness -- can be severe. These side effects can be worse for young women who haven't gone through menopause, she says.

The drugs can disrupt women's plans to have children, because women shouldn't get pregnant while taking them. Young women also are more likely to be caring for children as they go through treatment, making it harder for them to focus on their own health, says Eric Winer of Boston's Dana-Farber Cancer Institute.

So Hershman wasn't surprised that the youngest patients in her study were the least likely to complete therapy. Although everyone in her study was covered by Kaiser Permanente, she notes that younger women in general are more likely to be uninsured or lose their coverage. Aromatase inhibitors can cost hundreds of dollars a month.

Hershman says women should never stop taking a prescription without first talking to a doctor, who may be able to help alleviate side effects or find financial help.

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