According to a study by Canadian researchers, women taking tamoxifen for their breast cancer and Paxil, an anti-depressant for their depression should avoid taking them together, as Paxil reduces the effecitiveness of Tamoxifen, thereby increasing their risk of dying.

According to lead researcher Dr. David Juurlink, Division Head of Clinical Pharmacology and Toxicology at Sunnybrook Health Sciences Centre, Toronto, long-term use of Paxil in combination with Tamoxifen, deprives women of Tamoxifen benefits.

Patients on tamoxifen needing an anti-depressant should probably be given something other than Paxil.

A selective serotonin re-uptake inhibitor (SSRI), Paroxetine or Paxil significantly inhibits cytochrome P450 2D6, an enzyme needed to for metabolising tamoxifen into its active form. However, researchers said certain other SSRIs like citalopram (Celexa) and venlafaxine (Effexor) that were evaluated, did not produce the same dampening effect.

According to Juurlink, all patients taking Paxil and Tamoxifen are advised to discuss changing their anti-depressant with their doctors, but must not discontinue Paxil, abruptly.

Doing so would be dangerous, due to withdrawal syndrome and the risk of depression becoming severer, while transitioning to another anti-depressant gradually over several weeks.

The report has been published in the online edition of the British Medical Journal.

For the study, researchers looked at the medical records of 2,430 women with breast cancer between 1993 and 2005, all taking Tamoxifen, with 30% of them also taking anti-depressants, Paxil being the commonest. Ofter, anti-depressants are prescribed not only for easing symptoms of depression, but for reducing hot flashes associated with tamoxifen.