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.
Traffic law policymakers, forensic toxicologists, and regular cannabis users who drive.
THC >2 ng/mL after 6 days; >5 ng/mL after 1 day of abstinence
What the researchers 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.
Why it 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 numbers in context
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 the study worked
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.
What this study cannot 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.
How to read the evidence
Moderate: systematic approach but only 6 studies met criteria, with small samples.
When this study was published
Published 2020.
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.
Questions still open
- Should per se THC limits be replaced with impairment-based testing? What blood THC level actually correlates with driving impairment in frequent users?
Common questions
How long does THC stay in your blood?
Can you fail a THC driving test while sober?
Read the original research
"Residual blood THC levels in frequent cannabis users after over four hours of abstinence: A systematic review.".
Drug and alcohol dependence, 216, 108177
Citation
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
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