WHY IT MATTERS: Patients who use cannabis medicinally need to understand that subjective feelings of being “fine to drive” can diverge significantly from actual measured driving performance, particularly in the hours following consumption. CLINICAL OVERVIEW: The relationship between cannabis consumption and driving performance is genuinely complex, because THC impairment does not follow the same linear dose-response curve that alcohol does, and standard roadside testing fails to capture the nuanced ways cannabis affects reaction time, lane tracking, and divided attention. Tolerance, consumption method, cannabinoid ratios, and individual pharmacokinetics all influence how significantly any given person is impaired behind the wheel.
THC levels in blood and urine are "unreliable" indicators of driving impairment – leafie
WHY IT MATTERS: Patients who use cannabis medicinally could face legal consequences for impaired driving based on biological thresholds that do not accurately reflect whether they were actually impaired at the time of driving. CLINICAL OVERVIEW: The relationship between THC concentration in biological fluids and actual driving impairment is far more complex than a simple number can capture. THC is highly lipophilic, meaning it distributes rapidly into tissues and does not remain in blood proportionally to psychoactive effect, which makes blood levels a poor proxy for functional intoxication.