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Tamoxifen Label May Finally Warn About Antidepressant Risks

Jun 2, 2009 | Parker Waichman LLP

A study that we reported on yesterday that showed that use of some antidepressants-namely Paxil, Prozac and Zoloft-may interfere with the efficacy of tamoxifen, leading to recurrence of breast cancer, has the Food & Drug Administration (FDA) planning to warn doctors of potentially adverse interactions.  According to Dow Jones News Wire, it will likely add the information on the interactions to tamoxifen's label.

Tamoxifen is used to prevent the return of estrogen-dependent breast tumors.  More than 500,000 women in the U.S. are taking tamoxifen and about 30% of those women are also prescribed antidepressants to treat hot flashes, Dow Jones News Wire said.

Hot flashes are a common side effect of tamoxifen therapy.  As we reported yesterday, doctors started using antidepressants for hot flashes after hormone-based treatments were linked to the development of breast cancer. However, treating tamoxifen hot flashes with antidepressants is an off-label use of such drugs.  

As we reported yesterday, a study conducted by Medco Health Solutions Inc. and presented over the weekend at the American Society of Clinical Oncology's annual meeting, found breast cancer patients who took an antidepressant like  Paxil, Prozac or Zoloft with tamoxifen were more than twice as likely to have their cancer return.  The study, which involved 1,300 women, found that those who took such drug combos for one  year had a breast cancer recurrence rate of 16% compared to a recurrence rate of 7.5% for women not taking the drugs. However, according to Dow Jones News Wire, not all antidepressants had the same impact,  and the study showed women on drugs like Celexa, Lexapro and Luvox didn't have a statistically higher rate of cancer recurrence.

According to a report on, tamoxifen works by combining with  the  CYP2D6 enzyme,  which is produced by the liver.  This produces endoxifen, a substance  known to fight tumors. But some antidepressants, including Paxil, Prozac and Zoloft, block the  CYP2D6 to varying degrees, which likely impacts the efficacy of tamoxifen.

Prior to the Medco study, lab studies had shown that some antidepressants could impact tamoxifen. As we reported yesterday, the FDA had already taken a look at interactions between tamoxifen and antidepressants. According to, the 2006 review concluded that data on the effect should be added to tamoxifen labeling; however, to date, there is no such statement on this risk on the medication’s label.

According to Dow Jones News Wire, Larry Lesko, director of the FDA's Office of Clinical Pharmacology, said now that the FDA has evidence about clinical outcomes as a result of the drug interaction, it will likely add that information to tamoxifen's label.

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