#78 Strong Clinical Relevance
High-quality evidence with meaningful patient or clinical significance.
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.
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. Unlike alcohol, where blood concentration correlates reasonably well with impairment, cannabis pharmacokinetics vary dramatically based on frequency of use, individual metabolism, tolerance, and the presence of other cannabinoids.
“Basing roadside impairment law on THC blood levels is the scientific equivalent of arresting someone for speeding based on how much gas is in their tank.”
🧠 Cannabis impairment testing remains a significant clinical and legal challenge, as THC blood and urine concentrations correlate poorly with actual driving performance or cognitive impairment levels.
💊 Unlike alcohol, which shows predictable dose-response relationships with impairment, THC’s effects depend on individual tolerance, consumption method, and cannabinoid profile, making any fixed threshold inherently unreliable.
🩸 This evidence gap creates real clinical implications: patients using cannabis therapeutically may test positive without impairment, while detection windows vary dramatically (hours for blood, weeks for urine).
🔹 Until validated behavioral or performance-based testing becomes standard, clinicians should counsel patients about individual responsiveness and advise against driving when subjectively impaired, regardless of test results.
💬 Join the Conversation
Have a question about how this applies to your situation? Ask Dr. Caplan →
Want to discuss this topic with other patients and caregivers? Join the forum discussion →
Have thoughts on this? Share it: