pretty much everything I've read about DXM has said that DXM and acetaminophen are a bad combo, and that an overdose of acitomenophen can cause severe liver damage or death. from the DXM FAQ:

3.4 What Happens if I Drink the Wrong Cough Syrup?

If it has gauifenesin (Robitussin DM and generic "DM" syrups have this drug), you'll probably puke your guts up. If it has acetaminophen (paracetamol), you stand a good chance of a painful, prolonged death. Anything else, you're likely to ruin the trip and may end up in the hospital.

just something to keep in mind before (or (god forbid) while) you guzzle down that bottle of nyquil...