A decrease in driving reaction times and performance occurs approximately 5 hours after smoking cannabis which is more pronounced and lasts longer in occasional users than frequent users

In March 2019 researchers from France published the results of their study to assess the association between THC and driving risk. A total of 30 individuals, aged 18 to 34 years, were involved in the study; 15 were frequent cannabis users (1-2 joints/day) and 15 occasional users (1-2 joints/week). Each individual had blood taken 12 times over a 24-hour period before and after inhalation of placebo or THC 10 mg or 30 mg with reaction times being assessed 7 times using psychomotor vigilance tests as well via a driving simulator. Results showed that the highest blood level of THC was twice the level in frequent users than in occasional users for a same dose and occurred 5 minutes after the end of consumption. THC remained detectable only in frequent users after 24 hours. Despite a standardized consumption, frequent users consumed more THC from each cigarette regardless of dose. Driving reaction times were found to be decreased in both groups of users but the effect of THC on driving performance was more marked in occasional users than in frequent users. These effects peaked at around 5 hours after administration, and the duration was longer in occasional users than in frequent users.

Hartley S et al. Effect of Smoked Cannabis on Vigilance and Accident Risk Using Simulated Driving in Occasional and Chronic Users and the Pharmacokinetic-Pharmacodynamic Relationship. Clin Chem. 2019 Mar 14. [Epub ahead of print]

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