A new study performed by researchers from the Institute for Clinical Evaluative Sciences and Sunnybrook Health Sciences Center claims that women that were being treated for breast cancer were more likely to die if they were taking Paxil than those that didn’t take the drug.

“These results highlight a drug interaction that is extremely common, widely underappreciated and potentially life-threatening, yet uniformly avoidable,” writes researcher David Juurlink.

The research is based on data gathered from 2,430 women ages 65 or older who were treated with the breast cancer drug tamoxifen between 1993 and 2005. Of these women, 30 percent also were taking an antidepressant at the same time.

Those that were taking Paxil with the tamoxifen treatment saw between a 25 and 91 percent higher rate of death — depending on the duration of the drugs’ overlap in treatment. While it isn’t completely clear as to why this happened, it is believed that Paxil directly blocks the effects that tamoxifen has on the patients.

If the tamoxifen is going to work, the body has to be able to process it with an enzyme called CYP2D6. However, SSRIs like Paxil can block what the enzyme does for these women that are on the treatment.

“Paxil is a fairly potent inhibitor of that enzyme,” Juurlink says.

While Prozac likely has the same effect on tamoxifen as Paxil does, most of the women studied didn’t take that drug. Paxil is the leading SSRI on the market. While this study has shown that women taking Paxil are more likely to die from breast cancer, it still is not recommended for them to stop taking the drug without consulting a doctor because Paxil also is well known to cause severe withdrawal symptoms.

If you currently are taking both of those drugs and want to stop, it is best to consult your doctor first and work out a weaning process. That is the safest way to come off of Paxil and any other antidepressant.