Paxil, a commonly-prescribed antidepressant, may interfere with the breast cancer drug tamoxifen, Canadian researchers have found. The interaction appears to increase the risk the cancer could return, the study suggests.

The research, conducted on Ontario women with breast cancer, found that those who took Paxil, the brand name of paroxetine, or generic versions, while also taking tamoxifen were more likely to die of their breast cancer.

The longer the women took both drugs, the more likely they were to die, the researchers report in the British Medical Journal.

Dr. David Juurlink, a scientist at the Institute for Clinical Evaluative Sciences in Toronto and a co-author of the study, says it appears that Paxil blocks the body's ability to process tamoxifen, thereby negating its effects.

"Tamoxifen on its own doesn't do very much," Juurlink explained to CTV's Canada AM Tuesday.

"It has to be turned by the liver into the metabolite endoxifen. And as it turns out, paroxetine turns that critical step off -- and it turns it off very effectively."

To conduct the study, Juurlink and fellow researchers at ICES analyzed health records for 2,430 Ontario women aged 66 to 75 who were treated with tamoxifen between 1993 and 2005.

Tamoxifen t works by blocking the effects of estrogen, which fuels the growth of estrogen receptor-positive breast cancer tumours. The drug has been shown to reduce breast cancer recurrence by up to 50 per cent if women take it for five years.

About 30 per cent of the women in the study were also taking an antidepressant, with Paxil being the most common. The researchers found that those taking Paxil had a markedly increased risk of breast cancer death.

Fifteen per cent of the patients died of breast cancer during the study. After other factors were taken into the account, the researchers found that women who took Paxil for a quarter of the time they were also taking tamoxifen were 25 per cent more likely to die of breast cancer.

This rose to a worrying 91 per cent risk for the women who took tamoxifen and Paxil together for 75 per cent of the time.

The researchers estimate that taking both drugs for the entire time of treatment results in one extra death from recurrent breast cancer at five years for every seven women treated.

Paxil is part of a family of antidepressant medications called selective serotonin re-uptake inhibitors, or SSRIs. They include such popular drugs as fluoxetine (Prozac), bupropion (Wellbutrin) and sertraline (Zoloft).

It's not uncommon for women diagnosed with breast cancer to be prescribed a drug from the SSRI class to treat the depression and anxiety that comes with breast cancer diagnosis, or to reduce hot flashes, which are a common side effect of tamoxifen.

While the researchers in this study did not find a higher risk of death from breast cancer among women who took Prozac and Wellbutrin, Juurline says that doesn't mean they're safe.

"We didn't find it with the others but there's an important caveat here and that's that there's good reason to think that Prozac might carry the same risk. We just didn't find it in our study, probably because not enough people were taking it," he explained.

Both Prozac and Wellbutrin are already known to interfere with the metabolism of tamoxifen and likely should also be avoided.

Juurlink notes that although the study didn't focus on younger women, there's no reason to believe the same results wouldn't be found in younger ages. He says his study focused on older women only because that was the age group with the most data available.

"It calls for speculation, but there's no reason to believe the results wouldn't be applicable to younger patients," he said.

He cautioned that the results of his study don't mean that women taking tamoxifen should panic and stop taking their antidepressants.

"Certainly, don't stop taking your Paxil suddenly. There's a well-described withdrawal syndrome that actually has potentially dangerous consequences," he warned.

He advised that women talk to their doctors about switching to other antidepressants.

"I think for women taking these drugs together, there are better options that don't cause the same problem," he noted. "A transition to one of those drugs should be gradual and made over two or three weeks."

He noted that although Paxil appears to negate the effects of tamoxifen, the antidepressant itself doesn't increase the risk of cancer.

Juurlink says more study is now needed on how other medications in the SSRI class affect tamoxifen, as well as how different age groups are affected.