In a new study they found that certain anti-depressants (Prozac, Paxil or Zoloft) can wipe out the benefits of using tamoxifen:

Breast cancer survivors risk having their disease come back if they use certain antidepressants while also taking the cancer prevention drug tamoxifen, worrisome new research shows.
About 500,000 women in the United States take tamoxifen, which cuts in half the chances of a breast cancer recurrence. Many of them also take antidepressants for hot flashes, because hormone pills aren’t considered safe after breast cancer.
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Breast cancer recurred in about 7 percent of women on tamoxifen alone, and in 14 percent of women also taking other drugs that could interfere — mainly the antidepressants Paxil and Prozac, and, to a lesser extent, Zoloft.