Women are who take both tamoxifen for breast cancer treatment and a certain antidepressant may face an increased risk of death, according to investigators from the Institute for Clinical Evaluative Sciences (ICES) in Toronto.

Background information provided in the press release announcing the findings revealed that antidepressant are of particular importance because they are commonly used in women with breast cancer, often for long periods of time.

To study the effects of paroxetine, a selective serotonin reuptake inhibitor (SSRI), on tamoxifen, Catherine Kelly, MD, and colleagues examined the health care reports of 2 430 women ≥ 66 years of age with breast cancer who received tamoxifen between 1993 and 2005. Among the 30% of women who received an antidepressant at the same time during their treatment with tamoxifen, paroxetine was the most commonly used agent. 

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Researchers reported that when used in combination with tamoxifen, paroxetine, but not other SSRIs, resulted in an increased long-term risk of breast cancer death, which correlated with the extent when both drugs were used simultaneously. Based on the results of the study, the researchers estimate that treatment with paroxetine for 41% of the total time on tamoxifen will result in 1 additional breast cancer death at 5 years for every 20 women so treated.

“Our findings indicated that the choice of antidepressant can significantly influence survival in women receiving tamoxifen for breast cancer,” said David Juurlink, MD, one of the study’s authors and a scientist at ICES. “When co-prescription of tamoxifen with an antidepressant is necessary, preference should be given to antidepressants that exhibit little or no impact on tamoxifen’s metabolism.”

The study’s findings were published in BMJ (2010;340:c693).