Tamoxifen May Up Risk of Second, More Aggressive Bre...(hed online Aug. 25 in the journal Cance...),Health

Exactly why the tamoxifen boosts the risk of ER-negative second cancers is not known, but Li said it could be that prolonged tamoxifen use provides "a competitive advantage for the growth of ER-negative breast cancer cells."

Other experts agreed that the study is no reason to give up on tamoxifen. "The thing we have to remember is tamoxifen saves lives," said Dr. Victor Vogel, national vice president of research at the American Cancer Society.

"Even though the risk of ER-negative goes up, only with five years or more, the risk of ER-positive [second breast cancer] goes down," he said. "We shouldn't lose sight of that."

"I worry people are going to panic if they are taking this drug," said Dr. Minetta Liu, director of translational breast cancer research at the Georgetown University Medical Center, in Washington, D.C. But they should not, she said, because "it has helped a huge number of women."

More information

To learn more about tamoxifen, visit the National Cancer Institute.

SOURCES: Victor Vogel, M.D., national vice president, research, American Cancer Society, Atlanta; Christopher Li, M.D., Ph.D., associate member, Fred Hutchinson Cancer Research Center, Seattle; Minetta Liu, M.D., director, translational breast cancer research, Georgetown University Medical Center, Washington, D.C.; August 25, 2009, Cancer Research