A review of 52+ studies found that while acute cannabis use typically impairs cognition and psychomotor function, blood THC levels do not correlate well with impairment level, making alcohol-style per se driving limits scientifically problematic.
Policymakers, law enforcement, traffic safety researchers, and medical cannabis patients who drive.
THC levels ≠ impairment
What the researchers found
Marijuana use is associated with significant cognitive and psychomotor effects. Blood THC levels do not correlate well with impairment level, unlike blood alcohol. Acute infrequent cannabis use typically causes impairment, but this is not consistently the case for chronic heavy users. Cannabis driving laws vary widely across states.
Why it matters
As more states legalize, cannabis driving enforcement needs a scientific foundation. This review highlights the fundamental problem: unlike alcohol, there is no reliable biomarker-impairment relationship for cannabis.
The numbers in context
28 epidemiological studies, 16 acute cognitive studies, 8 chronic studies reviewed. Cannabis is a commonly found substance in DUI cases. Blood THC does not correlate with impairment. Chronic heavy users may not show consistent impairment.
How the study worked
MEDLINE search reviewing 28 epidemiological studies, 16 acute cognitive/psychomotor studies, 8 chronic studies, and relevant state and federal laws.
What this study cannot tell us
Review-level evidence. Studies vary in methodology, cannabis potency, and impairment measures. The distinction between acute/infrequent and chronic/heavy use adds complexity. State laws continue to change rapidly.
How to read the evidence
Rated moderate because the review synthesizes a substantial body of evidence from multiple study types.
When this study was published
Published in 2019.
The bigger picture
The cannabis driving policy challenge reflects a broader problem: applying alcohol-era enforcement tools to a substance with fundamentally different pharmacokinetics. New approaches (performance testing, combined biomarkers) may be needed.
Questions still open
- Should cannabis DUI enforcement abandon blood level thresholds? Would performance-based field testing be more reliable? How should chronic medical cannabis users be handled in driving enforcement?
Common questions
Can you set a legal THC driving limit like alcohol?
Does cannabis impair driving?
Read the original research
Driving Under the Influence of Cannabis: A Framework for Future Policy.
Anesthesia and analgesia, 128(6), 1300-1308
Citation
Chow, Robert M; Marascalchi, Bryan; Abrams, Winfred B; Peiris, Nathalie A; Odonkor, Charles A; Cohen, Steven P. (2019). Driving Under the Influence of Cannabis: A Framework for Future Policy.. Anesthesia and analgesia, 128(6), 1300-1308. https://doi.org/10.1213/ANE.0000000000003575