- Tamoxifen is a drug prescribed to women to shrink breast cancers or to prevent breast cancer from developing in women who are considered to be at a higher risk. A study published in the Journal of the National Cancer Institute conducted by the Indiana University School of Medicine, chronicled on the American Cancer Society's website, examined the use of this drug in combination with paroxetine, which is sold under the brand name, Paxil (see link in References). Paxil is an antidepressant that has been prescribed for some women with breast cancer to treat side effects (such as hot flashes) associated with the disease as well as the mental effects of cancer. According to an article published on HealthCentral.com, more than 1/6 of women with breast cancer taking Tamoxifen also take either Paxil or Prozac, another antidepressant (see link in References). The study examined 12 women to discover if diminished benefits were seen from taking these drugs in combination, specifically if a breast cancer recurrence was seen.
- When ingested, the body converts tamoxifen into an active substance known as endoxifen, which then affects breast cancer cells. However, the same enzymes used to break down Tamoxifen, such as one called CYP2D6, are used to break down Paxil and make it effective.
Researchers at the Indiana University School of Medicine found that women who take Paxil and Tamoxifen together have a lower level of endoxifen, at a 56 percent lower rate than women who take Tamoxifen alone, according to the American Cancer Society. However, this was not a similar level across the board: Some women have higher levels of endoxifen while others had a lower level. Researchers concluded that some women are able to metabolize both Tamoxifen and Paxil when taken together while others are not. - An additional study conducted by Matthew P. Goetz, M.D., at the Mayo Clinic College of Medicine found that women who had suppressed levels of CYP2D6--either due to taking Paxil or an additional genetic deficiency--had a higher rate of breast cancer recurrence than those who did not. On a 10-year basis, this rate was a 60 percent recurrence for CYP2D6-suppressed women compared with a 20 percent recurrence rate for those who did not.
While physicians do not recommend altering treatment recommendations for Paxil and Tamoxifen just yet, this evidence may signal the need for more testing in women who take the drug in combination as well as the development of new drugs that require different enzymes to metabolize than Tamoxifen.