NEW YORK (Reuters Health) — About one third of women with breast cancer who are being treated with tamoxifen stop taking the medication before the end of the recommended five years of therapy, a study shows.

This is a concern, noted Dr. Thomas I. Barron in an interview with Reuters Health given that “the maximum benefit from tamoxifen is gained when it is taken for five years.”About 75 percent of women diagnosed with breast cancer are prescribed a hormonal drug, such as tamoxifen, primarily to reduce the risk of the cancer returning in the future.

Barron, from Trinity College Dublin, and colleagues studied adherence to tamoxifen using prescription refill data in 2,816 women. They report in the journal Cancer that 22 percent of the women had stopped taking the drug by one year, and 28 percent by two years.

By 3.5 years, 35 percent had stopped taking tamoxifen, without switching to an alternative therapy.

“Younger women (35 to 45 years old) and older women (older than 75 years) were more likely to stop their tamoxifen,” Barron told Reuters Health. “Also women who had been prescribed antidepressants in the past — an indicator that they may have a history of depression — were more likely to discontinue treatment.”

“Women discontinuing their tamoxifen early may not receive the full benefits of their treatment as these are closely related to the length of time tamoxifen is taken for,” Barron cautioned. “The benefits of five years of tamoxifen have now been shown to extend for at least 15 years after a diagnosis of breast cancer.”