Tamoxifen and low dose aspirin (original
link)
Here is a screen shot that shows the impact of aspirin on endoxifen (this is the chemical tamoxifen is converted to that actually prevents breast cancer recurrence). Half of the endoxifen is converted by a liver enzyme called UGT2B7 to something our body can than eliminate.
Aspirin is a moderate inhibitor of UGT2B7 so it keeps the endoxifen in the body longer before it is eliminated. This could actually be beneficial for 2D6 IMs as other threads have pointed out. Of course, studies would have to be done to confirm this. There is no current medical information that indicates any reason to be concerned about reducing tamoxifen effectiveness with aspirin.