The abstract Hopeful posted discusses serotonin and BC. It says "We found that serotonin slightly induced cell proliferation in estrogen receptor positive MCF-7 breast cancer cells. Because autophagy is implicated as a survival pathway in some cancers we also examined if serotonin induces autophagy in MCF7 ER+ breast cancer cells. We found that serotonin induces autophagy"
So..the suggestion seems to be that supplementation of serotonin (action of some antidepressant meds) levels might encourage ER+ breast cancer cell growth and survival.
The issue of whether drugs that are CYP 2D6 inhibitors (again, some anti depressants) interfere with Tamoxifen treatment is a separate issue...one that seems to ebb and flow based on pubmed searches.
So..seems like might be wise to only use antidepressants judiciously at this point.
On the other hand, there was some early research suggesting that Prozac might increase effects of chemo:
http://her2support.org/vbulletin/showthread.php?t=41742
The main researcher involved with that seems to have been pulled into different research.