SUNDAY, June 2 (HealthDay News) -- When it comes to using the drug tamoxifen to prevent breast cancer's return, longer may be better for some patients, a new study finds.

Women combating estrogen-sensitive breast tumors fared better when treated with 10 years of tamoxifen compared to those given the current standard of five years, researchers found.

However, longer tamoxifen regimens were associated with a greater risk for side effects from the cancer-suppressing drug, such as night sweats, hot flashes, blood clots, strokes and a heightened risk for cancer of the lining of the uterus (endometrial cancer).

Still, the investigation found that 10 years of treatment gave patients better protection against the recurrence of estrogen receptor (ER)-positive breast cancers and/or death than did treatments offered for half as long.

Assessing the findings in conjunction with the results of another recently completed study called the ATLAS trial, the team concluded that the benefits of prolonged therapy still outweigh any risks.

The study was scheduled for presentation Sunday in Chicago at the annual meeting of the American Society of Clinical Oncology (ASCO).

Five years of tamoxifen "is already an excellent treatment but we thought that longer treatment might be even better because women with ER-positive breast cancer can have recurrences long after treatment is completed," study lead author Richard Gray, professor of medical statistics at the University of Oxford in the United Kingdom, said in an ASCO news release.

"Until now, though, there have been doubts whether continuing tamoxifen beyond five years is worthwhile," he added. "This study and its international counterpart ATLAS confirm that there is definitely a survival benefit from longer tamoxifen treatment and many doctors will likely recommend continuing tamoxifen for an extra five years."

The research was funded by Cancer Research UK and the UK Medical Research Council. Findings presented at medical meetings are typically considered preliminary until published in a peer-reviewed journal.

The study involved nearly 5,000 British breast cancer patients, all of whom had already completed a five-year regimen of tamoxifen by the time they entered the study at some point between 1991 and 2005.

In turn, half the women were randomly assigned to continue the medication for another five years, while half stopped the treatment altogether.

The result: no appreciable difference was found in death or breast cancer recurrence when assessing the impact of the sixth to ninth years of continued treatment.

However, when looking long-term -- at the 10th year and beyond -- the team found that the longer treatment option was having an impact. For example, breast cancer recurrence was lower among the 10-year group than the five-year group, at 16.7 percent versus 19.3 percent, respectively.

What's more, continuing treatment for double the usual period also reduced the overall risk of dying from breast cancer, the team noted.

"These results are therefore practice-changing for premenopausal women with hormone receptor-positive breast cancer and especially relevant for women who are at high risk of recurrence," Dr. Sylvia Adams, an ASCO spokesperson and breast cancer expert, said in the ASCO news release.

More information

There's more on estrogen and cancers at the U.S. National Cancer Institute.

SOURCE: American Society of Clinical Oncology, news release, June 2, 2013