A common antidepressant weakens or cancels the beneficial effects of a standard treatment for breast cancer, according to a study released today.

Researchers said that women who took the mood drug paroxetine - better known by its brand names Paxil and Seroxat - at the same time as the breast cancer medication tamoxifen faced an increased risk of death.

The antidepressant alone had no impact on the course of the disease but should not be taken at the same time as the anti-cancer drug, they said.

Tamoxifen significantly improves survival for the dominant type of breast cancer, the most commonly diagnosed cancer in women worldwide.

But to work properly, it must be converted by the liver into an active metabolite, the chemical that remains after a drug is broken down by the body.

It was previously suspected that antidepressants that boost the release of the naturally occurring neurotransmitter serotonin, such as paroxetine, might interfere with this process.

To find out, Catherine Kelly and colleagues at the Institute for Clinical Evaluative Sciences in Toronto, Canada, investigated the healthcare records of 2430 women with breast cancer who had received tamoxifen from 1993 to 2005.

About 30 per cent of these women also received an antidepressant - paroxetine more than any other - at the same time, often prescribed over long periods to help patients cope with the stress of battling the deadly disease.

The use of paroxetine in combination with tamoxifen resulted in an increased long-term risk of breast cancer death - about one additional fatality after five years of illness for every 20 women treated.

"These results highlight a drug interaction that is extremely common, widely underappreciated and potentially life-threatening, yet uniformly avoidable," the study's co-author, David Juurlink, said.

Tamoxifen is prescribed to women who have so-called oestrogen-receptor positive breast cancer, which accounts for up to 80 per cent of all cases of the disease.