Time profile of serum THC levels in occasional and chronic marihuana smokers after acute drog use – implication for drivind motor vehicles

Summary

Cannabis consumption has individual influence to cognitive and psychomotor functions of drivers and it has been generally accepted that driving under influence is risky in the perspective of traffic safety. However, rules how to assess fitness to drive are not quite clear. The psychoactive compound delta-9-tetrahydrocannabinol (THC) impairs cognition, psychomotor behaviour and driving performance in a dose-related manner approximately. After a single drug dose, THC blood concentration peaks within minutes, before the end of smoking, with a subsequent rapid decrease to the analytical limit of detection. Peak euphoria is delayed compared to THC peak blood concentration and physiological and behavioural effects return to baseline within 3-5 hours. In chronic users, the lipophilic THC accumulates in fat tissues, where its slow redistribution into blood is the rate limiting process in its terminal elimination. In our experimental study we have attempted to contribute to this discussion with results obtained from human volunteers – cannabis consumers in Czech Republic. Aim of our study was to document the time profile of serum THC level in occasional and chronic cannabis users. The observational interval covered the time immediately after the drug consumption (an own cigarette/joint) till 24 hours after. Our preliminary results have shown that in occasional users, THC serum levels cannot be detected already 4 hours after usual cannabis dose, whereas in chronic users measurable THC concentrations in serum persist longer. Moreover, some chronic consumers were practically with permanent THC detection during our observation period and also the chronic users consumed higher THC doses significantly related to doses in occasional ones. Presented results of the experimental study with human volunteers confirm a great individual variability of the kinetic profile of THC in blood due to complicated redistribution. The practical forensic question is how long the psychotropic effects of THC can persist after the last drug dose. In chronic users there are well documented indications of long term adverse effects to neurocognitive functions. THC blood level itself can not directly document the intensity of impairment of a driver. Moreover, the concentration of THC in blood at the time of driving is probably substantially higher than at the time of blood sampling. Therefore due to the prevention of traffic risk, some countries adopted per se traffic legislation based on analytical principle with minimum tolerance to illegal drugs in blood of drivers at driving. Low blood concentrations of THC close to the limit of detection of a specific toxicological method (GC-MS or LC-MS) are justified in an effective traffic legislation.

Keywords:
cannabis – delta-9-tetrahydrocannabinol (THC) – psychotropic effects – pharmacokinetics – traffic risk – driving impairment