Breast Cancer Health Center

Tamoxifen: Risk of Rare Second Breast Cancer?

Aug. 25, 2009 -- A new study links long-term use of the breast cancer drug tamoxifen to a rare but aggressive form of breast cancer, but experts say the findings shouldn't stop breast cancer patients from taking tamoxifen.

"We don't think that it overall changes the risk-benefit equation, in that women who are eligible to take this drug probably should still take it because of its proven benefit," researcher Christopher Li, MD, PhD, an associate member of the Fred Hutchinson Cancer Research Center in Seattle, tells WebMD.

"I think the worst thing that could happen, on a public health basis, with this paper is for patients and their doctors to look at this and say, 'Oh, this is a reason not to take tamoxifen.' Nothing could be further from the truth, for the reason that it obviously has enormous benefit," says Victor Vogel, MD, MHS, the American Cancer Society's national vice president for research.

Here's a look at the study Li and Vogel are talking about -- and why they stand by tamoxifen's use in breast cancer patients with "ER positive" breast cancer.

Most breast cancers are " ER positive," or estrogen receptor-positive. That means they grow when exposed to the hormone estrogen. Tamoxifen and other breast cancer hormone therapies thwart ER-positive breast cancer cells. 

"ER negative" breast cancers, on the other hand, are rarer, tend to be more aggressive, and are more difficult to treat. They're not treated by tamoxifen or another class of estrogen-related breast cancer drugs called aromatase inhibitors, because ER-negative breast tumors aren't sensitive to estrogen.

Tamoxifen Study

Li's team studied data on nearly 1,100 Seattle-area women aged 40-79 who were treated for ER-positive breast cancer between 1990 and 2005. The group included 367 women who developed breast cancer in their other breast at least six months after their first diagnosis.

Li and colleagues interviewed all of the women and checked their medical records, noting any use of tamoxifen or other hormone therapies to help prevent breast cancer's return, and how long those drugs were used.

Most of the women took tamoxifen -- aromatase inhibitors are newer drugs and weren't available during many of the years studied. And most of the women didn't have another cancer develop in their other breast.

Women who used tamoxifen or other breast cancer hormone therapies were 60% less likely to develop an ER-positive cancer in their other breast, compared to those who never took tamoxifen.

But women who used tamoxifen for five or more years were about four times more likely than women who never used breast cancer hormone therapies to develop an ER-negative tumor in their other breast.

Tamoxifen use for less than five years wasn't linked to ER-negative breast cancer risk. The study didn't include any women taking tamoxifen to try to prevent a first breast cancer.