After Germany raised its legal THC driving limit, half of impaired drivers fell below the new threshold while half of unimpaired drivers exceeded it

Median THC levels in administrative and criminal traffic offenses were identical (3.44 ng/mL). After legalization, THC medians rose modestly (to 3.95-3.97 ng/mL). Under the new 3.5 ng/mL threshold, 58.3% of occasional users with potentially impaired driving fell below the limit (escaping sanctions), while almost half of frequent unimpaired drivers exceeded it. Only 7.4% of frequent users benefited from the raised threshold as intended. Drivers under 21 decreased from 18.2% to 13.3% of cases.

Retrospective analysis of 48,058 traffic-related blood samples containing THC from three German federal states between April 2021 and March 2025. Cases were submitted for toxicological analysis under administrative or criminal traffic offense statutes. Four consumption categories were established based on THC-carboxylic acid concentrations.

Germany's 2024 cannabis legalization included raising the driving THC limit, intended to protect frequent users who separate use from driving. This study provides the first systematic evaluation showing the new threshold may not achieve its intended goals.

This is one of the first large empirical evaluations of a post-legalization THC driving threshold change. The finding that blood THC levels poorly distinguish impaired from unimpaired drivers challenges THC-based per se limits as a regulatory approach.

Laboratory data from three states may not represent all of Germany. Cannot directly measure impairment at time of driving. Consumption categories based on metabolite ratios are approximations. Short post-legalization follow-up period.