Tamoxifen and Paxil: What You Need to Know
February 17, 2010 "Tamoxifen and Paxil a Bad Mix"
"Paxil Reduces Tamoxifen's Effectiveness"
"Paxil and Tamoxifen May Cause Death"
Your probably saw one of these headlines—or something similar—this week.
They were the result of a study published in this week's British Medical Journal.
There's just one thing many reporters failed to note in their stories: these findings weren't totally new news at all. In fact, "Paxil and Tamoxifen: A Bad Mix?" was the headline we used for a piece posted about a study published in the Journal of the National Cancer Institute back in December 2003. At the time, scientists had already started to worry that tamoxifen and Paxil might be a bad combination. They were concerned because Paxil, as well as some other antidepressants, inhibit an enzyme called CYP2D6 (enzymes are proteins that speed up chemical reactions in the body). And tamoxifen needs to be converted into 4-hydroxy-tamoxifen (the form that the body can use) by this enzyme.
The advice given in 2003: Women who were on tamoxifen should switch from Paxil (sold generically as paroxetine) to another antidepressant.
[We also now know that some women have a variation of the CYP2D6 that results in their body not metabolizing tamoxifen properly. A genetic test, called the "tamoxifen resistance" or "CYP2D6" test can tell you if you have this genetic variation.]
Concerns about combining Paxil and tamoxifen were again raised in June 2009 at the American Society of Clinical Oncology Annual Meeting. As I blogged from the meeting, a study presented at ASCO suggested that women who were on SSRIs had an increased risk of a breast cancer recurrence, while a second, smaller study suggested they did not. (Neither of these presentations was by the same research group whose study made headlines this week.)
The advice given then: Women who were on tamoxifen should switch from Paxil to another antidepressant, and should also be aware that there are other drugs that may also interfere with tamoxifen metabolism.
What's different about the study that made headlines this week? In this study, the researchers didn't look at risk of recurrence but risk of death. And while there was an increased risk, it was not as shockingly big as the headlines made it out to be.
First, it's important to point out that this study did not compare a group of women who were on tamoxifen with a group of women who were on tamoxifen and an antidepressant. Rather, it looked at women who were taking an antidepressant and were on tamoxifen for 2.5-5 years between 1993 and 2005 and looked at death rates based on the type of antidepressant the woman was using.
The study included 7489 women, all of whom were 66 years of age or older (the majority were over 76). By the end of the follow-up period, there were 1074 women who had died; 374 of these women died of breast cancer. When looked at by type of antidepressant used, the data suggested that if a group of 20 women used Paxil for about 40% of the time they were on tamoxifen, there would be one additional breast cancer death after five years than there would have been if these women had not been taking Paxil.
What should you do? If you are currently taking Paxil and tamoxifen, you should not stop taking Paxil. Instead, you should call your doctor to get a prescription for a different antidepressant and to find out the best way to change from one to another. Don't just stop taking your Paxil. If you are taking Prozac, you may also want to consider switching to another drug as some studies have suggested it might interfere with tamoxifen metabolism as well.
If, in the past, you took tamoxifen and were on Paxil and are now worried, I'd say try to think about this: There were 630 women in the study who were on Paxil and tamoxifen. During the study 105 of them died of breast cancer and 191 died of another cause. In other words, most of the women did not die of breast cancer.
What can you do now? You might want to pay even more attention to the few things we know can reduce your risk of recurrence: not gaining weight, eating a well-balanced diet, and exercising as often as you can.
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