Paxil blocks effect of breast cancer drug
from the Toronto Star
February 9, 2010 HEALTH REPORTERA popular antidepressant eradicates the benefits of life-saving breast cancer drug tamoxifen, a new Ontario study shows.
The study of 2,430 women taking tamoxifen showed the drug's potent cancer-fighting capacity was lost in patients using paroxetine anti-depression drugs such as Paxil.
"If someone is on tamoxifen for five years ... and they're taking paroxetine the entire time, they've effectively not been on tamoxifen," said study co-author Dr. David Juurlink, a researcher with the Institute for Clinical Evaluative Sciences.
"And what this study shows for the first time is that that translates into an increased risk of deaths because the patients are being deprived of a very important drug."
The study was published Monday in the British Medical Journal.
Conducted between 1993 and 2005 on women 66 and older, it showed that those treated with Paxil-like drugs for 40 per cent of the time they were taking tamoxifen had a significantly greater risk of dying of breast cancer within five years of diagnosis.
This combination resulted in one additional death for every 20 patients so treated, the study showed.
Juurlink said about 30 per cent of women taking tamoxifen were on antidepressants, and most of those were prescribed Paxil-like drugs.
While other antidepressants, most notably Prozac and Wellbutrin, can also interfere with tamoxifen's effectiveness, there are others that won't, Juurlink said.
"There are many other options out there that ... are almost certainly better choices for women who are receiving tamoxifen ... It's not a reason to withhold antidepressants."
Women taking Paxil with their tamoxifen should not stop the antidepressant abruptly, Juurlink said.
"There's a well-recognized withdrawal syndrome for people who stop it suddenly, so if it's going to be discontinued it needs to be tapered off over a couple of weeks," he said.
An email from Paxil maker GlaxoSmithKline Canada said the company had been aware of reported interactions between the two drugs and had posted a warning on the antidepressant's monograph.
"GSK will review these additional data and will work with regulatory authorities to determine next steps," the email said.