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    Antidepressants and Tamoxifen May Not Mix

    Study Shows Some SSRIs May Cause Breast Cancer Drug to Lose Effectiveness
    WebMD Health News
    Reviewed by Louise Chang, MD

    June 1, 2009 (Orlando, Fla.) -- Women who are taking the breast cancer drug tamoxifen should avoid certain antidepressants, researchers say.

    That's because a large U.S. study showed that certain selective serotonin reuptake inhibitor (SSRI) antidepressants may prevent tamoxifen from working properly.

    In the study, women on tamoxifen who were also taking one of three SSRIs -- Paxil, Prozac, or Zoloft -- were about twice as likely to have their cancer come back, compared with women on tamoxifen who were not taking an SSRI.

    SSRIs are prescribed not only to treat the depression that often plagues women with cancer, but also to help relieve hot flashes that can be caused by tamoxifen.

    Tamoxifen mixes with a liver enzyme called CYP2D6 in the body to change into the tumor-fighter endoxifen. Many SSRIs block the same enzyme, thus preventing tamoxifen from morphing into its active form.

    "We've known that these [drugs] block the activation of tamoxifen chemically, but this is the first time there's evidence that they may be decreasing the effectiveness of their anticancer treatment," says researcher Robert Epstein, MD, chief medical officer at Medco Health Solutions Inc., a pharmacy benefits management company.

    Paxil, Prozac, and Zoloft are categorized as moderate-to-potent SSRIs because they are strongest in their ability to block the CYP2D6 enzyme. Celexa, Lexapro, and Luvox are considered weaker SSRIs.

    The weaker SSRIs were not linked to an increased risk of breast cancer recurrence, the study showed.

    The findings were presented at the annual meeting of the American Society of Clinical Oncology.

    A second, Dutch study presented at the meeting found no increased risk of breast cancer coming back in women taking tamoxifen and either weak or strong SSRIs. But the number of women taking both drugs in that study was small -- reason enough for women to err on the side of caution, doctors say.

    "There are other options [besides the stronger SSRIs], so why not use them?" says Judy R. Gralow, MD, a breast cancer specialist at the University of Washington in Seattle.

    Gralow tells WebMD that if an antidepressant is needed, she recommends her patients take Effexor. "We have a lot of information on Effexor's effectiveness against hot flashes and depression, so that's the one I use," she explains.

    Gralow moderated a news conference to discuss the findings of both studies.

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