Blood THC Levels Do Not Reliably Predict Driving Impairment, Systematic Review Finds
A blood test can show that a driver has THC in their system, but it may not reliably show whether that person is impaired behind the wheel. That is the central finding of a 2025 systematic review published in the International Journal of Neuropsychopharmacology.
Researchers from the Centre for Addiction and Mental Health, Health Canada, Thomas Jefferson University and other institutions examined 4,845 records before identifying 12 peer-reviewed studies that directly evaluated the relationship between blood delta-9-THC concentrations and driving performance. Most of the research involved simulated driving in controlled laboratory settings.
Ten of the 12 studies found no statistically significant linear relationship between blood THC levels and driving measures. The outcomes included lane-position variability, often called weaving; speed; car-following behavior; reaction time; and overall driving performance.
The review does not mean that cannabis has no effect on driving. Cannabis can impair skills relevant to safe driving, including coordination, attention, reaction time and decision-making. The U.S. Centers for Disease Control and Prevention notes that acute cannabis use is associated with motor-vehicle crashes, while also cautioning that it is difficult to connect a particular THC concentration with an individual driver’s level of impairment. The safest choice is not to drive after using cannabis.
What the review questions is the assumption that impairment rises in a predictable, alcohol-like pattern as blood THC increases. Unlike blood alcohol concentration, THC levels can change rapidly after consumption and can vary substantially among individuals. Factors such as timing, route of administration, frequency of use, product potency and individual physiology can all affect the relationship between a blood measurement and observed performance.
The two studies that did report significant associations between blood THC and driving performance used more complex driving scenarios. Those findings suggest that elevated THC may be more closely related to poorer performance when drivers must manage demanding or unexpected situations, although the evidence remains limited. A driver may appear to compensate in a relatively simple task yet struggle when confronted with distractions, sudden hazards or multiple demands at once.
That distinction is important for interpreting both the research and cannabis-driving laws. The review did not determine whether any particular state statute is legally valid, nor did it establish a substitute threshold for identifying impairment. Rather, it indicates that a fixed blood-THC cutoff should not automatically be treated as equivalent to a specific degree of driving impairment.
This concern is not new. The National Highway Traffic Safety Administration’s report to Congress on marijuana-impaired driving likewise concluded that the presence or concentration of THC in a driver’s body is not a reliable measure of impairment. NHTSA distinguishes between evidence that cannabis can affect driving and the separate challenge of determining impairment from a toxicology result alone.
State approaches differ. Some jurisdictions use impairment-based laws, while others apply zero-tolerance rules, per se limits or evidentiary thresholds. NHTSA’s current guidance explains that, for cannabis and other drugs, the relationship between a measured drug concentration and driving impairment has not been established in the same way it has for alcohol.
The review’s authors call for larger studies that systematically vary driving complexity, cannabis potency and other conditions. Future research may need to combine toxicology with behavioral observations, validated roadside assessments and information about when and how cannabis was consumed.
For now, the findings support a cautious conclusion: blood THC can provide evidence of cannabis exposure, but it should not be mistaken for a precise impairment meter. Cannabis-related driving enforcement and public-safety policy will need to account for both the real risks of impaired driving and the substantial limitations of relying on a single blood concentration.
