Cannabis in road traffic - a retrospective analysis to identify possible cut-off-values

In light of the recent partial legalization of cannabis in Germany and the associated adjustment of the legal threshold for THC serum concentration from 1 ng/ml to 3.5 ng/ml, we investigated whether a specific THC serum concentration or cannabis influence factor (CIF) value could be identified, above which behavioural and driving impairments significantly increase. We retrospectively analysed cases from 2020 to 2021, in which blood samples had been submitted for toxicological analysis from drivers suspected of operating a vehicle under the influence of cannabis alone (DUIC) or in combination with alcohol (DUIAC). 740 cases were evaluated with respect to behavioural and driving impairments, serum concentration of THC and its metabolites, CIF values, and-in cases involving co-consumption-blood alcohol concentration (BAC). Our findings indicate that cannabis impairs psychomotor and cognitive functions and that THC serum concentrations are roughly correlated with driving impairment. However, we were unable to define a clear cut-off-value above which a significant increase in impairment could be detected. Slightly (but statistically significant) higher THC serum concentrations and CIF values were observed in cases where there was concrete evidence of criminal offenses pursuant to § 316 of the German Penal Code. However, absolute differences were small and there was too much overlap between the two groups to define a useful cut-off-value for THC. There is a measurable relationship between THC serum concentration and impairment. This relationship is too weak to scientifically define a cut-off value. Regarding the revised legal threshold for THC serum concentration, we found that approximately one-third of the evaluated cases would no longer be subject to legal penalties if the new threshold of 3.5 ng/ml were applied.