Gamma-hydroxybutyric acid (GHB) is endogenously produced within the central nervous system, however it is also used as a medication for the treatment of a variety of clinical conditions, sold under the name Zyrem in the United States and Alcover in Europe. It is a very dangerous drug with a very limited safety margin, and is classified as a controlled substance in many countries. The interpretation of post-mortem studies of GHB concentrations is problematic; GHB can be detected in urine and blood from non-GHB users, both before and after death, and concentrations in both matrices may rise with prolonged storage. Because it is produced as a post-mortem artifact, forensically defensible cut-offs for post-mortem blood concentrations have yet to be established. Given the enormous degree of inter and intra-individual variation in GHB production that has been documented, it is unlikely they ever will. The important issue for forensic scientists is whether the detection of GHB in urine, in concentrations above some yet to be determined value, can be used as evidence for drug facilitated assault. In an attempt to see if a cut-off level could be determined we analyzed urine from 39 alcoholics who were being treated with known oral doses of Alcover (group 1), and compared the results with concentrations found in the urine of 30 volunteers who had no exogenous GHB intake (group 2), and 30 urine specimens taken from the alcoholics before they initiated GHB therapy (Alcover treatment group 3). More than one third (36.6%) of subjects being treated with GHB were found to have urinary GHB concentration that fell between 2.75 and 10 microg/mL. The data suggests that caution must be used when applying the currently used cut-off of 10 microg/mL.