Paxil and Tamoxifen May Cause Breast Cancer Relapse or Death: Study

Canadian researchers are reporting that the antidepressant Paxil may interfere with breast cancer drug treatments, increasing the risk of relapse and death in breast cancer survivors.

The new study, conducted by scientists and doctors at the Institute for Clinical Evaluative Sciences and the Sunnybrook Health Sciences Center in Toronto, was published in the latest issue of the British Medical Journal. Researchers found that selective serotonin reuptake inhibitors (SSRI), like Paxil, can interfere with tamoxifen, an estrogen-based breast cancer therapy drug.

Paxil (paroxetine) is a selective serotonin reuptake inhibitor by GlaxoSmithKline PLC prescribed to treat depression. Approved in 1992, it has become one of the most commonly prescribed drugs in the United States, with sales of just under $1 billion last year. It is also available as a generic.

Researchers looked at 2430 women age 66 or older who had been treated with tamoxifen for breast cancer between 1993 and 2005, and who were also taking Paxil or another SSRI at some point during that time period. The study’s results indicate that the drug reduces or completely mitigates the effectiveness of tamoxifen, confirming scientists’ suspicions that Paxil inhibits tamoxifen’s bioactivation.

About 1.5 million women worldwide are expected to be diagnosed with breast cancer this year, researchers say. It is the most commonly diagnosed form of cancer for women. Experts estimate that about 25% of women dealing with breast cancer experience a depressive disorder. Many of those women are prescribed an antidepressant.

“Our findings have major implications for clinical practice, particularly in light of the frequency of combination therapy,” researchers said. “In our study, 30% of women who started tamoxifen treatment also received antidepressants, and paroxetine was the most commonly used SSRI.”

The study comes as Glaxo tries to deal with hundreds of lawsuits from plaintiffs claiming that Paxil causes suicidal tendencies and birth defects. The company has reportedly already spent more than half a billion dollars to settle Paxil lawsuits, and still faces hundreds more.

There are currently about 600 more Paxil birth defects lawsuits pending. Following a jury award in October for $2.5 million in one Paxil birth defect trial in Pennsylvania, Savvas Neophytou, a London-based Panmure Gordon analysts, told investors that Glaxo faced a potential liability of $1.5 billion in the cases.