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.