Frequent cannabis users can have blood THC above legal driving limits even after days of abstinence
A systematic review of 6 studies found that frequent cannabis users may have blood THC above 2 ng/mL after six days of abstinence and above 5 ng/mL after one day, challenging per se driving limits.
Quick Facts
What This Study Found
Across 6 independent studies, frequent cannabis users showed blood THC above 2 ng/mL (or plasma THC above 3 ng/mL) after six days of abstinence in 5 studies. Two studies found blood THC above 5 ng/mL (or plasma above 7.5 ng/mL) after one day of abstinence. This means frequent users could fail legal THC driving limits while completely unimpaired.
Key Numbers
1,612 articles screened; 6 studies included; 5 found THC >2 ng/mL after 6 days abstinence; 2 found THC >5 ng/mL after 1 day abstinence.
How They Did This
Systematic review searching MEDLINE, EMBASE, PsycINFO, and Web of Science for studies reporting THC levels in frequent cannabis users after more than 4 hours of abstinence. 1,612 articles screened; 6 independent studies met criteria.
Why This Research Matters
Many jurisdictions use 2 or 5 ng/mL blood THC as per se driving limits. If frequent users exceed these thresholds days after last use, unimpaired drivers are being criminalized for a positive test that reflects prior use, not current impairment.
The Bigger Picture
THC-based per se limits were modeled on alcohol, where blood levels correlate reliably with impairment. THC behaves differently: it accumulates in fat tissue and releases slowly. This pharmacological reality undermines the scientific basis of THC per se driving laws.
What This Study Doesn't Tell Us
Only 6 studies met criteria (limited evidence); definitions of "frequent use" varied; did not assess actual impairment alongside THC levels; small samples in most included studies; blood vs plasma THC complicates comparisons.
Questions This Raises
- ?Should per se THC limits be replaced with impairment-based testing?
- ?What blood THC level actually correlates with driving impairment in frequent users?
Trust & Context
- Key Stat:
- THC >2 ng/mL after 6 days; >5 ng/mL after 1 day of abstinence
- Evidence Grade:
- Moderate: systematic approach but only 6 studies met criteria, with small samples.
- Study Age:
- Published 2020.
- Original Title:
- "Residual blood THC levels in frequent cannabis users after over four hours of abstinence: A systematic review.".
- Published In:
- Drug and alcohol dependence, 216, 108177 (2020)
- Authors:
- Peng, Yuan Wei, Desapriya, Ediriweera, Chan, Herbert(5), R Brubacher, Jeffrey
- Database ID:
- RTHC-02774
Evidence Hierarchy
Analyzes all available research on a topic using a structured method.
What do these levels mean? →Frequently Asked Questions
How long does THC stay in your blood?
For frequent users, blood THC can remain above 2 ng/mL for at least six days and above 5 ng/mL for at least one day after last use, according to this review. Occasional users clear THC much faster.
Can you fail a THC driving test while sober?
Yes. This review shows that frequent cannabis users can exceed legal per se THC limits days after their last use, when they are no longer impaired. The authors suggest this means per se THC limits may not accurately identify impaired drivers.
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Cite This Study
https://rethinkthc.com/research/RTHC-02774APA
Peng, Yuan Wei; Desapriya, Ediriweera; Chan, Herbert; R Brubacher, Jeffrey. (2020). "Residual blood THC levels in frequent cannabis users after over four hours of abstinence: A systematic review.".. Drug and alcohol dependence, 216, 108177. https://doi.org/10.1016/j.drugalcdep.2020.108177
MLA
Peng, Yuan Wei, et al. ""Residual blood THC levels in frequent cannabis users after over four hours of abstinence: A systematic review.".." Drug and alcohol dependence, 2020. https://doi.org/10.1016/j.drugalcdep.2020.108177
RethinkTHC
RethinkTHC Research Database. ""Residual blood THC levels in frequent cannabis users after ..." RTHC-02774. Retrieved from https://rethinkthc.com/research/peng-2020-residual-blood-thc-levels
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.