This seems as good a place as any to mention something I found somewhat surprising over the past 6 or 12 months. I started doing DXM again on occasion. As it used to, my speech was mangled and I was obviously intoxicated (very scarily so to those uninformed), but clearly not drunk, to any who came in contact with me within the next 16-24 hours. Sometimes longer if I dosed slowly or redosed a bit (I imagine this effect comes more from DXO than DXM).
From experimenting with ondansetron / zofran to offset nausea, I decided to try triple my usual dose (4 mg -> 12 mg), taking them a couple minutes apart, the ondansetron first. I tried this from reading about the novel interactions between the two (and possibly MXE) as reported by users like ps00donym and others. I have read elsewhere that both it (ondansetron / zofran) and DXM are metabolized by the CYPD26 enzyme.
This post by user thenightwatch quotes a handful of ps00donym/psood0nym/dude-man's posts.
Anyway, long story short, my speech remains nearly immaculate throughout the entire experience. And within 8 hours from dosing I'm clear as day. I'm not sure the mechanism behind this but it seems to be a consistent effect every time I take ondansetron with it. Something or other to do with CYPD26 I'm willing to bet. Seeing as how most of these chemicals affect speech similarly, I would say this is worth further exploration for you committed psychonauts.
By the way I noticed no other unexpected reactions from combining the two. It took away nausea, some stomach cramps, lots of the side effects like heat flashes and itching, and cleared up my speech to the point where I could do DXM on weekdays where I had work the following morning. I was more worried about pupils than my speech thanks to the damn near miraculous difference. For those of you with garbled speech, please try this out and report back. Perhaps a thread should be made with this explicit topic for it to be properly assessed.