DXM is a NMDA Receptor Antagonist just like PCP and Ketamine. Meaning it could plausibly produce a false positive for those drugs. Unless by some amazing happestance you have a script for Ketamine which I'm %99.999 is on the Misuse of Drugs Act even though I've heard otherwise your screwed there. PCP you'd be overly screwed. Have fun with DXM though! Great trip!