Women prescribed tamoxifen to prevent a recurrence of breast cancer should avoid taking the antidepressant Paxil and its generic equivalents because of a potentially dangerous drug interaction, a study suggests.

Paxil, or paroxetine as it is known generically, appears to negate the effects of tamoxifen, and may lead to an increased risk of death from breast cancer, the researchers say.

Tamoxifen is prescribed as a treatment and preventive therapy for estrogen receptor-positive breast cancer.

Researchers at Institute for Clinical Evaluative Sciences in Toronto analyzed health and prescription records for 2,430 Ontario women aged 66 to 75 who were treated with tamoxifen between 1993 and 2005 to prevent breast cancer recurrence.

The women were also taking antidepressant medications known as selective serotonin reuptake inhibitors, or SSRIs. They included such popular drugs as paroxetine (Paxil), fluoxetine (Prozac), and sertraline (Zoloft). SSRIs are typically prescribed for women on tamoxifen to treat depression and anxiety or to reduce hot flashes, a common side-effect of tamoxifen.

Paroxetine, taken by more than a quarter of the women in the study, had a negative effect on survival that the other SSRIs appeared not to share, the researchers determined.

"What we found was that using paroxetine ... in combination with tamoxifen increased the risk of death due to breast cancer," said Dr. David Juurlink, a coauthor of the study published in this week's British Medical Journal.

"I think there's a strong argument to be made for transitioning to a different antidepressant," he added.