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Cannabis use produces dose-related impairments in cognitive and behavioral functions that may induce serious consequences in a number of circumstances such as driving (1,2). Ag-Tetrahydrocannabinol (THC) and its primary metabolite, 11-hydroxy-Ag-tetrahydrocannabinol (11-OH-THC) are the active constituents responsible for the occurrence of these adverse effects. An important forensic issue is to know how long impairment persists. Some studies indicate that the effects of cannabis may still be present after THC blood concentrations have dropped to a few nanograms per milliliter (3), but this has never been confirmed by biological data. In order to answer this question, a new approach could consist of analyzing cannabinoids in brain and comparing the results obtained with those in the corresponding blood. To date only one paper has reported THC concentrations in brain, however this study was performed in only one case. In addition, metabolites were not analyzed, and the precise location of brain sample was not defined (4). In this study, paired samples (blood and brain) were provided from 12 postmortem cases in which cannabinoids were detected in blood. THC, 11-OH-THC, and 11 nor-9carboxy-THC (THC-COOH) were determined by gas chromatography-mass spectrometry (4). Brain samples (about 200 mg) were homogenized in pH 7.4 Tris buffer, and then extracted as blood samples were. In the first step, 10 cases were analyzed for THC only, and the precise location of brain sampling was not defined. These results, expressed in nanograms per gram, are indicated in Table I. No correlation was observed between blood and brain concentrations of THC. In all cases, THC was found in brain at significant concentrations as compared with those reported by Kudo et al. (5). THC concentrations in brain were higher than THC concentrations in blood in all cases. In three cases (8-10) THC was still present in brain, whereas it was not detected in blood (limit of detection, 0.2 ng/mL). These results could explain the recent findings of Herning et al. (6), revealing that cerebrovascular resistance and systolic velocity are significantly increased in marijuana users after a month of monitored abstinence. Taking into account these results, 11-OH-THC and THC-COOH were also analyzed in two cases where the location of brain samples were defined. The mean concentration (n = 3) obtained in blood and in several brain areas are presented (Table II). These results indicate that THC was present at significant concentrations in all brain Table I. Cannabinoids Concentrations in Human Brain and Blood Samples

THC Can Be Detected in Brain While Absent in Blood*