Antidepressant blocks tamoxifen benefits

Research published in the British Medical Journal (BMJ) shows that the antidepressant paroxetine can interefere with the effectiveness of the breast cancer drug tamoxifen.

Tamoxifen significantly improves survival in breast cancer. However, the drug works when the liver converts tamoxifen into an active metabolite (endoxifen). Paroxetine is a member of the selective serotonin reuptake inhibitor (SSRI) class of drugs, which inhibit the metabolic step that converts tamoxifen to endoxifen.

These findings are significant as many women with breast cancer are prescribed antidepressants – often for long periods of time.

The researchers examined the healthcare records of 2,430 women aged 66 years or older with breast cancer who received tamoxifen between 1993 and 2005. Around 30 per cent of these women also received an antidepressant at some time during their treatment with tamoxifen, with paroxetine being the most commonly used agent.

The results showed that use of paroxetine, but not other SSRIs, in combination with tamoxifen, was associated with an increased long-term risk of breast cancer death.

"Our findings indicate that the choice of antidepressant can significantly influence survival in women receiving tamoxifen for breast cancer," says Dr David Juurlink, one of the study's authors and a scientist at the Institute for Clinical Evaluative Sciences (ICES).

"This observation is consistent with what we know about tamoxifen's metabolism. These results highlight a drug interaction that is extremely common, widely under appreciated and potentially life-threatening, yet uniformly avoidable."

However, the study's authors stressed that patients should not stop taking tamoxifen.