Neopine, a minor opium alkaloid and an isomer of codeine (also known as β-codeine), has been detected in both the urine of opium users and pharmaceutical codeine users. The characterization of neopine was achieved by comparison of the mass spectra and GC retention times of the trimethylsilyl derivative. The presence of neopine in the urine of pharmaceutical codeine users was attributed to the metabolism of codeine through a double bond migration in ring C, from the 7-8 to the 8-14 position. The potential use of the alkaloid as a confirmation marker of opium and/or pharmaceutical codeine use and the ability to differentiate these from heroin use has been discussed. © Springer-Verlag 2005.