Long-term breast cancer survivors may be alarmed by new risks associated with the commonly prescribed drug tamoxifen, uncovered by scientists studying the history of long-term tamoxifen use among some 1,100 female breast cancer survivors between ages 40-79 from 1990-2005.

Not unexpectedly, patients who took tamoxifen for at least five years enjoyed a reduced risk of a less-aggressive kind of second breast cancer (estrogen-receptor or ER positive breast cancer) compared to those receiving a placebo by 60 percent. Moreover, tamoxifen users were 40 percent less prone to develop a new tumor in the second breast.

On the downside, tamoxifen use by breast cancer survivors over the long term increased their risks of ER negative second cancer – tumors that weren’t estrogen-sensitive -- by a whopping 440 percent.

To the good, however, while these tumors were harder to treat, their occurrence was rare: Only one out of every seven patients developed them. What’s more, patients who had taken tamoxifen for four years or less weren’t affected at all.

Despite these latest findings, the American Cancer Society doesn’t advocate changing drug protocols or restricting use of the go-to breast cancer drug at all, noting, “The net benefit for tamoxifen is huge.”

Cancer Research August 25, 2009

Science Daily August 26, 2009

New York Times August 25, 2009

WebMD August 25, 2009