Current cannabis-impaired driving regulations lack scientific grounding because we still don't have adequate pharmacokinetic and pharmacodynamic data to connect THC blood levels to actual driving impairment.
Policymakers writing cannabis driving laws, researchers designing THC impairment studies, and drivers who use cannabis and wonder about legal limits.
What the researchers found
This commentary cuts to the heart of a policy problem: states are setting cannabis driving laws without the science to back them up. Some states use per se THC blood limits (you're "impaired" above a threshold), others use zero-tolerance approaches, and still others rely on observable impairment assessments. None of these approaches are well-validated.
The fundamental issue is pharmacokinetic—how THC moves through the body is far more complex than alcohol. THC is lipophilic (fat-soluble), meaning it's rapidly absorbed and redistributed into fat tissue, then slowly released back into the blood over hours to days. This creates a disconnect: blood THC levels drop quickly after use even while impairment may persist, and chronic users may have residual THC in their blood when fully sober.
The authors argue that the limited clinical data on THC pharmacokinetics and pharmacodynamics—how blood levels relate to actual cognitive and motor impairment—is the bottleneck. Without this data, any regulatory threshold is essentially arbitrary.
The call to action is directed at both researchers and policymakers: conduct comprehensive cannabis PK/PD studies using real-world products and dosing patterns, then use that data to develop and validate driving regulations.
Why it matters
Millions of people drive after using cannabis. Whether the legal framework for addressing this is based on science or arbitrary thresholds matters enormously—both for public safety (catching truly impaired drivers) and for justice (not penalizing unimpaired people who used cannabis days ago). The current situation serves neither goal well.
The numbers in context
THC blood levels drop rapidly after use but impairment may persist. Chronic users may test positive when fully sober. State regulations vary widely from zero-tolerance to per se limits to impairment-based assessment.
How the study worked
Commentary/narrative review analyzing the gap between current cannabis driving regulations and the scientific evidence needed to support them. Focuses on THC pharmacokinetics and pharmacodynamics as the missing link.
Who was studied
Not enough information to specify the population.
What this study cannot tell us
This is a commentary calling for research, not research itself. Doesn't propose specific THC thresholds. The challenge of establishing THC impairment thresholds may be inherently harder than for alcohol due to greater individual variability in cannabis pharmacology.
How to read the evidence
Expert commentary highlighting a research gap—valuable for framing the problem but doesn't provide new data.
When this study was published
Published in 2025 in Clinical Pharmacology and Therapeutics, reflecting the urgency created by ongoing cannabis legalization.
The bigger picture
This echoes RTHC-00171's argument about workplace testing thresholds in South Africa and connects to RTHC-00143's data on substance-positive injured drivers in Canada (54.9% tested positive for at least one substance). The same pharmacokinetic challenge applies across all THC testing contexts—driving, workplace safety, criminal justice. RTHC-00159's finding that cannabis dominates workplace drug tests further underscores the urgency of developing scientifically sound thresholds.
Replication
Not stated in abstract.
Funding
Not reported in abstract.
Conflicts of interest
Not reported in abstract.
Questions still open
- Is it even possible to set a meaningful per se THC limit given individual variability? Would cognitive testing (like roadside performance tests) be more scientifically valid than blood levels? How should regulations differ for smoked/vaped versus edible cannabis, given their different pharmacokinetic profiles?
Read the original research
Bridging THC Knowledge Gaps for Safer Roads: A Call for Action.
Clinical pharmacology and therapeutics, 118(3), 548-550
Clinical Pharmacology and Therapeutics is a reputable journal focused on drug therapy and pharmacology.
Citation
Li, Peizhi; An, Guohua. (2025). Bridging THC Knowledge Gaps for Safer Roads: A Call for Action.. Clinical pharmacology and therapeutics, 118(3), 548-550. https://doi.org/10.1002/cpt.3717