According to a summary posted by the Canadian Broadcasting Corporation, Canadian researchers looked at the health records of 2,430 women taking Nolvadex (tamoxifen) between 1993 and 2005. About 25 percent of the participants, or 630 women, were also taking Paxil (paroxetine).
The findings suggested that paroxetine interferes with the way tamoxifen works. Paroxetine is in a class of antidepressants known as selective serotonin reuptake inhibitors (SSRIs), which block or inhibit cytochrome P450 2D6, an enzyme needed to metabolize tamoxifen into its active form.
"Tamoxifen is an extremely important drug for breast cancer," said Dr. David Juurlink, a co-author of the study and a scientist at the Institute for Clinical Evaluative Sciences in Toronto. Dr. Juurlink added that Paroxetine "takes that benefit away by interfering with the body's normal handling of tamoxifen."
Separately the drugs work fine—and paroxetine, which is available as Paxil as well as under various generic names, continues to be effective when taken in combination with tamoxifen.
Tamoxifen is widely used to treat breast cancer and can be taken for up to five years in an effort to prevent a recurrence. The authors of the study stressed that their findings in no way should cause women to reconsider their use of tamoxifen. If anything, they should switch to a different antidepressant.
"If you need to be on an antidepressant with tamoxifen, there are choices, there are options out there that don't have the same risk," Juurlink said.
Doctors also stress that paroxetine should not be stopped cold turkey. Withdrawal should be done gradually and in consultation with a physician.
The researchers did not find an increased risk of death from recurrent breast cancer among the smaller sample of women on tamoxifen who took other SSRIs, such as fluoxetine (Prozac), sertraline (Zoloft), citalopram (Celexa) and venlafaxine (Effexor).
"Physicians should be aware that paroxetine and other strong 2D6-inhibiting drugs should be avoided in women treated with tamoxifen," said Dr. Frank Andersohn, a senior research associate at the Institute for Social Medicine, Epidemiology and Health Economics at Charité University Medical Center in Berlin, in an editorial accompanying the study.
In addition to treating depression, paroxetine eases the hot flashes that can occur following cancer treatment.