Analysis of 35 impaired driving cases with multiple blood draws showed that THC concentrations sometimes increased at later time points, making it difficult to determine when cannabis was last used.
Forensic toxicologists, defense attorneys in cannabis-DUI cases, policymakers setting THC driving limits
13.6% of samples showed THC increases at later time points
What the researchers found
In 35 DUID cases with multiple blood draws (81 total samples), THC concentrations ranged from 0.74 to 40 ng/mL. Notably, 11 samples showed an increase in THC concentration at a later collection time point, which undermines the assumption that THC levels consistently decline after use. Collection times ranged from 32 minutes to over 12 hours after the incident.
Why it matters
Many jurisdictions set per se THC limits for driving, similar to blood alcohol limits. This study demonstrates a fundamental problem with that approach: unlike alcohol, THC blood levels do not reliably decline in a predictable pattern, making it impossible to back-calculate the level at the time of driving.
The numbers in context
35 cases, 81 samples. THC range: 0.74-40 ng/mL. 11-OH-THC range: 1.0-16 ng/mL (60 samples). THC-COOH range: 7.1-470 ng/mL. Collection times: 00:32-12:42 hours post-incident. 11 samples (13.6%) showed THC increase at a later time point.
How the study worked
Retrospective analysis of cannabinoid-positive driving under the influence cases from 2019-2023 that had multiple blood draws. Samples were analyzed using LC-MS/MS with reporting limits of 0.5 ng/mL for THC, 1.0 ng/mL for 11-OH-THC, and 5.0 ng/mL for THC-COOH.
What this study cannot tell us
Relatively small sample (35 cases). Cannot control for subjects' cannabis use between blood draws in all cases. No information on the specific cannabis products used, route of administration, or chronic use history. Forensic casework data with inherent variability in collection timing and conditions.
How to read the evidence
Moderate evidence from real-world forensic casework data, though limited by small sample size and uncontrolled variables.
When this study was published
2025 study analyzing DUID cases from 2019-2023.
The bigger picture
As more jurisdictions implement cannabis-impaired driving laws, the scientific limitations of blood THC testing become increasingly important. This study provides real-world DUID casework evidence that even the gold-standard approach of multiple blood draws cannot reliably establish the time of cannabis use, challenging the validity of per se THC limits.
Questions still open
- What causes THC levels to increase at later time points? Is redistribution from fat stores responsible? Should per se THC driving limits be abandoned in favor of impairment-based assessments? How should forensic toxicologists interpret rising THC levels in casework?
Common questions
Can a blood test tell when someone last used cannabis?
Why do THC levels sometimes go up?
Read the original research
Changes in blood cannabinoid concentrations over multiple collection times in driving under the influence of drugs casework.
Journal of analytical toxicology, 49(8), 576-586
Citation
Peterson, Brianna L; Hessler, Meaghan R. (2025). Changes in blood cannabinoid concentrations over multiple collection times in driving under the influence of drugs casework.. Journal of analytical toxicology, 49(8), 576-586. https://doi.org/10.1093/jat/bkaf052
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