Tamoxifen May Up Risk of Second, More Aggressive Breast Cancer

Aug. 26 -- TUESDAY, Aug. 25 (HealthDay News) -- The drug tamoxifen is a tried-and-true way to lower the chances of developing the most common type of recurrence among breast cancer survivors, but new research suggests it raises the risk of getting a more aggressive cancer in the healthy breast by more than four times.

However, the finding is no reason to ditch the anti-cancer drug, as even the lead researcher emphasized.

"All treatments have risks and benefits," said study author Dr. Christopher Li, an associate member of the Fred Hutchinson Cancer Research Center in Seattle. "We know that the benefits of tamoxifen treatment clearly outweigh the risks. This study adds another risk, but doesn't change the overall balance. If you consider the full balance, for most women the benefits are going to far outweigh the risks."

Li and his colleagues evaluated tamoxifen use among 1,103 breast cancer survivors from the Seattle and Puget Sound area. The women were diagnosed initially with ER-positive breast cancer, the more common and less aggressive form, between the ages of 40 and 70.

Of those women, 369 developed a second breast cancer. Nearly all of those who took the hormonal therapy after surgery or other treatment used tamoxifen for five or more years. The researchers compared those who took tamoxifen with those who did not and found the drug, which blocks estrogen (which helps tumors grow) reduced ER-positive second breast cancers but boosted the risk of ER-negative tumors, which are less common but more aggressive.

"When we looked at the different types of second cancers, tamoxifen only lowered the risk of the more common and less aggressive type of cancer, ER-positive," Li said. "It lowered that by about 60 percent. But it increases the risk of developing ER-negative, the more aggressive cancer, by fourfold."

The study is published online Aug. 25 in the journal Cancer Research.

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."