N-Desmethyldoxepin or nordoxepin is the primary metabolite in urine and plasma of doxepin, a tricyclic antidepressant marketed as Apona, Adapine, or Doxal. This internal standard is suitable for numerous GC/MS and LC/MS applications from forensic analysis and clinical toxicology testing to urine drug testing, pain prescription monitoring and isotope dilution methods.