This study describes for the first time the direct involvement of serotonin in human breast cancer. Serotonin is most familiar in its role as a neurotransmitter involved with mood regulation, but it also has a role in normal mammary gland development, where it helps to regulate lactation and involution (the shrinkage of the milk-production apparatus when it is not needed). Excess serotonin is made in breast cancer cells, according to the study, where it functions improperly, contributing to malignant growth and inhibiting appropriate cell death. The net effect is that serotonin in breast cancer contributes to abnormal cell growth.

Concerns about antidepressants and risk of breast cancer

This study contributes to the developing understanding of the role of serotonin in breast cancer. Certain antidepressants such as Prozac (fluoxetine), Paxil (paroxetine), and Zoloft (sertraline) that affect serotonin levels have been suspected of increasing breast cancer risk. Concern was first raised in the late 1990s when several studies reported associations between use of antidepressants (including some that were not selective serotonin reuptake inhibitors (SSRIs) and higher risk of breast cancer. These studies were widely criticized and subsequent studies for the most part have found no such associations in large populations of women.

However, interest in this topic has continued, especially as knowledge of serotonin's role in normal breast functioning and development has expanded. Several case reports have been published concerning young women who developed malignant breast disease that appeared to be related to antidepressant treatment. However, a 2009 study that included 29,284 women did not find any changes in breast density associated with antidepressant initiation, continuation, or discontinuation. Recent studies focusing on risk of breast cancer by hormone receptor status have found some associations between SSRI antidepressant use and progesterone receptor negative breast cancer (ER-/PR- or ER+/PR-) in older women. While these results remain to be confirmed and explained, women who have already been diagnosed with one of these subtypes and who are taking or contemplate taking antidepressants might consider discussing the choice of antidepressant with their oncologists.

Antidepressants can influence tamoxifen treatment

Separate from whether antidepressant use heightens overall breast cancer risk is the question of how antidepressants may interact with breast cancer treatments. Based on one recent 2009 study, antidepressants used by some breast cancer survivors to reduce the hot flashes caused by taking tamoxifen could increase the risk of recurrence. This interaction appears to depend, in part, on the genetic makeup of the patients. Just as the effect of tamoxifen treatment varies widely among individuals, so too does the interaction between tamoxifen and SSRI antidepressants. Until this question is settled and vulnerable patients can be reliably identified, it makes sense for those taking tamoxifen to avoid Paxil, Prozac, and possibly other SSRIs.