Dextromethorphan (DXM) is an OTC antitussive that, at high recreational doses, acts as an NMDA-receptor antagonist producing PCP- and ketamine-like dissociation, earning the name 'Poor Man's PCP.' It is misused mainly by teens and young adults ('robotripping') and is a well-documented cause of PCP false positives on immunoassay.¹

DXM and its active metabolite dextrorphan are NMDA antagonists like PCP and ketamine, which underlies both its dissociative effects and its cross-reactivity: an unexpected PCP immunoassay positive can be DXM. Misuse has held steady among adolescents, and toxicity worsens when combination products add acetaminophen, antihistamines, or pseudoephedrine.²

LC-MS/MS measures DXM and its active metabolite dextrorphan; both are NMDA antagonists and can produce a PCP immunoassay false positive, so a specific test distinguishes DXM use from true PCP exposure.³ CYP2D6 poor metabolizers (about 10% of the population) or patients on CYP2D6 inhibitors (some SSRIs, tricyclics) clear DXM much more slowly (half-life 23 to 42 hours vs 3 to 4 hours), extending detection.

• Confirm an unexpected PCP immunoassay positive with LC-MS/MS; DXM is a common cause.¹
• Ask about OTC cough-and-cold product use, especially in adolescents and young adults.
• Assess for co-ingestant toxicity (acetaminophen, antihistamines, pseudoephedrine) in DXM misuse.
• Account for CYP2D6 status; poor metabolizers show prolonged detection and effects.
• Interpret levels as exposure, not dose or timing.