THC blood levels do not reliably predict driving impairment, creating a policy challenge
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.
Quick Facts
What This Study 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.
Key Numbers
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 They Did This
MEDLINE search reviewing 28 epidemiological studies, 16 acute cognitive/psychomotor studies, 8 chronic studies, and relevant state and federal laws.
Why This Research 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 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.
What This Study Doesn't 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.
Questions This Raises
- ?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?
Trust & Context
- Key Stat:
- THC levels ≠ impairment
- Evidence Grade:
- Rated moderate because the review synthesizes a substantial body of evidence from multiple study types.
- Study Age:
- Published in 2019.
- Original Title:
- Driving Under the Influence of Cannabis: A Framework for Future Policy.
- Published In:
- Anesthesia and analgesia, 128(6), 1300-1308 (2019)
- Authors:
- Chow, Robert M, Marascalchi, Bryan, Abrams, Winfred B, Peiris, Nathalie A, Odonkor, Charles A, Cohen, Steven P
- Database ID:
- RTHC-01984
Evidence Hierarchy
Summarizes existing research on a topic.
What do these levels mean? →Frequently Asked Questions
Can you set a legal THC driving limit like alcohol?
Unlike alcohol, blood THC levels do not correlate well with impairment. This makes alcohol-style per se limits scientifically questionable for cannabis.
Does cannabis impair driving?
Acute use by infrequent users typically impairs cognition and psychomotor function. However, chronic heavy users may not show consistent impairment, complicating enforcement.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-01984APA
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
MLA
Chow, Robert M, et al. "Driving Under the Influence of Cannabis: A Framework for Future Policy.." Anesthesia and analgesia, 2019. https://doi.org/10.1213/ANE.0000000000003575
RethinkTHC
RethinkTHC Research Database. "Driving Under the Influence of Cannabis: A Framework for Fut..." RTHC-01984. Retrieved from https://rethinkthc.com/research/chow-2019-driving-under-the-influence
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.