It's mentioned a few times on the forum that DXM
can give false positives for PCP
contains some anecdotes and a link to Erowid's document elaborating on the issue. To sum it up:
- there's no conclusive answer
- but false positives for PCP and opiates
may be possible
- these possibilities were neither decidedly proved, nor disproved yet (although this thread
suggests something different)
- detection period is assessed as "3-5 days" although it would be dependent on variety of factors ("frequency of use, metabolic rate, body mass, age, overall health, drug
tolerance, and urine pH of each individual")
Here is yet another thread on this subject: How Can I fail for PCP?
I don't know what could be the possible basis for DXM showing up as PCP, but for a laymen dextromethorphan
(C18H25NO) and phenylcyclohexylpiperidine (C17H25N) at least seem
somewhat similar (both being members of dissociatives
family of course).
As for opiates, DXM is analog of levorphanol and chemically is a very close relative of opiates. Whether this could be justification of false positives for opiates is beyond my knowledge.