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Feb. 9, 2010 -- Breast cancer patients who take the antidepressant Paxil at the same time as tamoxifen face an increased risk of dying from their disease, a new study shows.
The longer the women in the study took the widely prescribed breast cancer drug with Paxil, the greater their risk of death.
There has long been concern that certain antidepressants -- including Paxil, Prozac, and Wellbutrin -- block tamoxifen's effect on cancer.
None of the women in the study took Wellbutrin and not enough took Prozac to show an effect, study co-author David N. Juurlink, MD, PhD, tells WebMD.
But he says women who are taking tamoxifen should probably avoid all three drugs.
"There is a good pharmacologic reason to believe these drugs should not be used," he says. "Tamoxifen users who are already on one of these drugs should consider switching to another antidepressant, but this has to be done gradually."
Juurlink says the three drugs inhibit the ability of a key enzyme in the liver, known as CYP2D6, to convert tamoxifen to its active tumor-fighting form.
Other antidepressants, including Effexor and Celexa, are not considered potent CYP2D6 inhibitors.
Tamoxifen is used to suppress the estrogen that fuels tumor growth in some breast cancers. Antidepressants are widely prescribed to lessen common tamoxifen-related side effects, including hot flashes.
In a study reported last summer at the annual meeting of the American Society of Clinical Oncology, three selective serotonin reuptake inhibitor (SSRI) antidepressants -- Paxil, Prozac, and Zoloft -- were linked to an increased risk of breast cancer recurrence in tamoxifen users.
Women in the study who took tamoxifen with one of the drugs had twice the risk of having their cancer come back as women who did not take an SSRI.
Erik Stanek, PharmD, who worked on the study, tells WebMD that it is increasingly clear that SSRIs that are moderate to potent CYP2D6 inhibitors should not be prescribed with tamoxifen.
Stanek is senior director for research with the New Jersey-based health benefits company Medco Health Solutions.
"When you consider all the research together there is some disagreement, but a great deal of concordance that drugs that are weak 2D6 inhibitors or don't interact at all are better choices."
In the newly published study, Juurlink and colleagues analyzed the medical records of 2,430 women with breast cancer who took tamoxifen for an average of about four years between 1993 and 2005.
About 30% of the women also took an SSRI at some point while taking tamoxifen. Paxil was the most widely prescribed antidepressant, followed by Zoloft and Celexa.
Of the drugs examined, only the Paxil-tamoxifen combination was associated with an increased risk of death from breast cancer.
Treatment with Paxil for 41% of the time on tamoxifen, which was the average length of overlap, was found to result in one extra breast cancer death for every 20 women treated over five years of follow-up.
The study appears in the journal BMJ.