Don't take Mucinex DM. Dextromethorphan and tamoxifen are both substrates of CYP2D6. Mucinex D should be okay, as should regular Mucinex.
At the bottom of the page is a list of the more common CYP2D6 inhibitors, substrates and inducers.
http://en.wikipedia.org/wiki/CYP2D6