We finally have clinical confirmation of something we have been suspecting scientifically for a long time now: there are significant interactions between some depression meds and tamoxifen.

We can actually quantify these with testing for CYP2D6 enzyme activity. We know for the last few years that women metabolize tamoxifen to the active form through this liver enzyme system, but 10% of women lack enough activity in the enzyme to do the chemical reaction.

Some antidepressants also decrease the activity of this enzyme complex, so it's not a surprise that there are clinical consequences to these drug interactions.

This is validating to me as an oncologist, because quite a few tamoxifen patients also find themselves on antidepressants. Antidepressants can also be used to treat hot flashes, and this is yet another concern in this high-risk population.

Finally, I have railed on the aromatase inhibitors before in this blog. Studies suggest that the whole benefit of these very expensive medicines is because they circumvent the whole CYP2D6 system. There hasn't been a survival benefit shown, but the improvement in progression-free survival is very small over tamoxifen, suggesting that it's just the 10% of tamoxifen non-metabolizers that are benefitting.

http://www.webmd.com/breast-cancer/news/20100209/paxil-tamoxifen-may-be-...