Canadian researchers on Monday suggested that women with breast cancer who take both tamoxifen and the antidepressant Paxil are likely to have a large risk of relapsing and dying because Paxil reduces tamoxifen's effectiveness.

The longer the overlap between Paxil and tamoxifen, the more likely the patients were to die, they reported in the British Medical Journal.

"There is probably a better choice of antidepressants for women taking tamoxifen but (any change) should be done gradually with a doctor", said Dr. David Juurlink of the Institute for Clinical Evaluative Sciences and Sunnybrook Health Sciences Center in Toronto.

The researchers reasoned that Paxil, sold generically as paroxetine, reacts with the compound the body uses to process tamoxifen.

Juurlink and colleagues revealed to have analyzed the healthcare records of 2,430 breast cancer patients 66 or older who took tamoxifen between 1993 and 2005. The result suggested that nearly 30 percent of the patients took an antidepressant in addition as well during their treatment with tamoxifen, and paroxetine was the most common one.

Tamoxifen is revealed to reduce the risk of breast cancer returning by 50 percent if women take it for five years. However, the drug is being substituted by a newer class of drugs called aromatase inhibitors.