Mean THC blood concentrations in drivers suspected of impaired driving in Norway increased 58% between 2000 and 2010, suggesting increased cannabis exposure from higher-potency products.
Read this if you want to understand whether increasing cannabis potency actually leads to higher THC levels in users' blood.
58% increase in mean THC blood levels in impaired drivers over one decade
What the researchers found
Researchers analyzed THC blood concentrations from 1,747 drivers apprehended in Norway on suspicion of driving under the influence of cannabis between 2000 and 2010. Mean THC concentrations increased from 4.0 ng/mL in 2000 to 6.6 ng/mL in 2010, a 58% increase over the decade.
For comparison, mean blood alcohol concentrations in impaired drivers increased only 3% over the same period, and amphetamine concentrations increased 16%. The disproportionate rise in THC levels suggested that cannabis users were being exposed to more THC, likely due to increasing potency of available cannabis products.
The increase in blood THC concentrations was partially paralleled by an increase in the percentage of drivers judged as lightly impaired by examining physicians.
Why it matters
While cannabis potency has been documented to increase in many countries, this study was among the first to demonstrate that increased product potency translated into higher human exposure. Blood concentrations directly reflect what the body is actually absorbing, not just what the product contains.
The numbers in context
Mean THC blood concentration: 4.0 ng/mL (2000) to 6.6 ng/mL (2010), a 58% increase. Comparison: ethanol increased 3%, amphetamines increased 16%. Sample: 1,747 THC-only cases, 38,796 ethanol-only cases, 2,493 amphetamine-only cases.
How the study worked
Retrospective analysis of blood samples from drivers apprehended for suspected impaired driving in Norway between 2000 and 2010. Cases involving only THC (n=1,747) were compared to cases involving only ethanol (n=38,796) or amphetamines (n=2,493) to provide context for the THC trend.
What this study cannot tell us
The sample consisted of drivers suspected of impairment, not a representative population of cannabis users. Users of higher-potency cannabis might be more likely to be apprehended. The study could not control for whether users adjusted their consumption in response to higher potency (e.g., smoking less). Norwegian cannabis markets may not reflect global trends.
How to read the evidence
Large observational dataset spanning a decade, though limited to a specific population of apprehended drivers.
When this study was published
Published in 2013 with data through 2010. Cannabis potency has continued to increase in many markets since then.
The bigger picture
This study bridged two separate lines of evidence: rising cannabis potency and its real-world consequences. Knowing that THC content in cannabis products is increasing is important, but demonstrating that users are actually achieving higher blood levels confirms the public health relevance of potency trends.
Questions still open
- Have THC blood concentrations in impaired drivers continued to rise since 2010? Do higher blood THC levels translate directly to greater driving impairment? Are cannabis users in legal markets, where potency is labeled, better able to self-regulate their intake?
Common questions
Are people getting more THC in their system than before?
Does higher-potency cannabis lead to worse driving impairment?
Read the original research
Has the intake of THC by cannabis users changed over the last decade? Evidence of increased exposure by analysis of blood THC concentrations in impaired drivers.
Forensic science international, 226(1-3), 197-201
Citation
Vindenes, Vigdis; Strand, Dag Helge; Kristoffersen, Lena; Boix, Fernando; Mørland, Jørg. (2013). Has the intake of THC by cannabis users changed over the last decade? Evidence of increased exposure by analysis of blood THC concentrations in impaired drivers.. Forensic science international, 226(1-3), 197-201. https://doi.org/10.1016/j.forsciint.2013.01.017