Paxil Limits Effectiveness of Breast Cancer Drug Tamoxifen, Study Says

Tamoxifen, a medication prescribed to treat breast cancer, may be less effective for women who also take the antidepressant drug Paxil during their treatment and may also lead to an increased risk of death, according to a study published Monday in the British Medical Journal.

“Paxil can deprive women of the benefit of tamoxifen, especially when it is used in combination with tamoxifen for a long time,” said lead researcher Dr. David Juurlink, of the Institute for Clinical Evaluative Sciences and Sunnybrook Health Sciences Center in Toronto.

Juurlink and his research team examined the healthcare records of 2,430 breast cancer patients 66 years or older who took tamoxifen between 1993 and 2005. Women who took Paxil, generically called paroxetine, in combination with their tamoxifen treatment were 25 percent more likely to die of breast cancer.

Patients who are being treated with tamoxifen should probably opt for an alternative antidepressant treatment to Paxil, Juurlink said.

Paxil is in a class of medications called selective serotonin reuptake inhibitors (SSRIs), which dramatically inhibits a specific enzyme needed to use tamoxifen in its proper form. These results were similar for women who took fluoxetine (Prozac) during the study. However, this effect was not seen in other SSRIs such as citalopram (Celexa) and vanlafaxine (Effexor).

Even with the documented effects of combining tamoxifen and Paxil, Juurlink warned that stopping Paxil suddenly can cause more severe depression symptoms while patients go through withdrawal from their antidepressant treatment.

He suggested that patients switch to an antidepressant that is less likely to influence the effectiveness of tamoxifen.

Women who take tamoxifen for five years may decrease their risk of breast cancer relapse by 50 percent.

“These results highlight a drug interaction that is extremely common, widely underappreciated and potentially life-threatening, yet uniformly unavoidable,” Juurlink said. Additional information about drugs and drug side effects may be found on Drugwatch.com.