Tamoxifen May Increase Risk of Rare and Aggressive Breast Cancer

A new study indicates that long-term use of the breast cancer drug Tamoxifen can increase the risk that women will develop a rare and aggressive form of aggressive breast cancer.

The study, published in the online August 25 edition of Cancer Research, finds that while short-term use of Tamoxifen has definite and clear benefits for women who have had breast cancer and are trying to avoid a recurrence, users who take the drug for extended periods of time are four times as likely to have breast cancer return.

Researchers said the drug’s estrogen-blocking effects are very useful in preventing common, less aggressive cancer from returning, but long-term Tamoxifen side effects could aid in the growth of rare and more aggressive cancers that thrive when estrogen levels are low.

The study looked at 1,100 women who had been treated for breast cancer. From that group, 369 developed new tumors. When the researchers compared those women to the women who had stayed in remission, they found that women who took Tamoxifen for more than five years were four times more likely to develop the new forms of breast cancer, as well as being 60% less likely to develop more common, estrogen-dependent, breast cancer.

Tamoxifen, also known by the brand names Nolvades, Istubal and Valodex, was developed by AstraZeneca 30 years ago, and is widely available in generic form. Originally used to treat early-stage breast cancer, it has been used for the last ten years to also reduce the risk of cancer in women who are considered to have an increased risk of developing breast cancer. The drug works by acting against the effects of estrogen in breast tissue, preventing estrogen receptor-positive cancer cells from bonding with estrogen, which they need to grow.

Researchers said that their results should not discourage women from using Tamoxifen. The increased risk of breast cancer only appeared after a period of about five years of use, and most doctors prescribe the drug for five years or less.

The study comes on the heels of another study earlier this year that found Tamoxifen problems related to the use of some popular antidepressants, like Zoloft, Paxil and Prozac. A Medco Health Solutions study, released in late May, found that antidepressants known as selective serotonin reuptake inhibitors (SSRIs) more than doubled the chances of a breast cancer recurrence when used with Tamoxifen.

About half a million women take Tamoxifen in the United States, and that study raises serious concerns that a large percentage may be negating its effectiveness unknowingly. Nearly 30% of breast cancer survivors taking Tamoxifen use some form of antidepressant to treat hot flashes, which are a common side effect of Tamoxifen.