Can a positive THC test tell us whether someone was impaired behind the wheel? Research involving prescribed medical cannabis patients and regular cannabis users shows why the answer is more complicated than a laboratory number.
A positive test is not a driving test
The central question is not whether THC can affect driving—it can. The question is whether finding THC in someone’s body proves that person was impaired while driving. Research suggests that a laboratory concentration alone is not a reliable substitute for measuring actual driving performance.
40 prescribed patients: Arkell et al. (2023)
In a study of 40 medical cannabis patients using their usual prescribed products, researchers assessed attention, memory and other cognitive skills. They did not detect an overall decline across the tested measures, and two measures improved. These were cognitive assessments, not an on-road driving test. The study also cannot establish that every patient is safe to drive after dosing.
Driving simulator research: Manning et al. (2024)
A study of 40 patients assessed simulated driving after their prescribed cannabis products, with THC doses ranging from 1.13 to 39.18 mg. Tests occurred about 2.5 and 5 hours after use. Researchers did not observe substantial overall deterioration in the measured simulator outcomes. Because the study lacked a randomized placebo comparison, the results should not be interpreted as a guarantee of unimpaired driving.
Why steady-dose patients may respond differently
Repeated exposure can produce tolerance to some THC effects, but not necessarily all of them. Symptom relief and familiarity with an established dose may also affect measured performance. None of these factors makes a person immune to impairment, especially when dose, product or timing changes.
191 drivers: Marcotte et al. (2022)
In a randomized trial involving 191 regular cannabis users, researchers compared smoked cannabis containing 5.9% or 13.4% THC with placebo. Simulated driving performance worsened at 30 and 90 minutes. At approximately 4.5 hours, average performance was no longer significantly different from placebo. Crucially, blood THC concentration did not reliably predict the degree of driving impairment. This finding does not mean that everyone is unimpaired after 4.5 hours.
What does frequent use change?
Frequent users may show smaller changes in certain performance measures than occasional users, but findings vary by task and individual. The most defensible conclusion is that dose, timing, tolerance and the individual all matter. No single result establishes universal driving safety.
Presence-based rules versus impairment-based assessment
Some jurisdictions set THC concentration thresholds or zero-tolerance rules. Those policies are distinct from approaches that require evidence of actual driving impairment. Because THC may remain detectable after its strongest acute effects have passed, a positive result does not independently establish how well a person was driving. Better validated assessment tools remain an important research need.
Maryland: why this research matters locally
Maryland does not have a fixed blood-THC per se driving limit comparable to its blood-alcohol threshold. The Maryland Cannabis Public Health Advisory Council addressed the limitations of THC detection as a marker of impairment in January 2026. Johns Hopkins has registered a driving-simulator research protocol with a planned enrollment of 90 participants (NCT06293040). The protocol includes cannabis-and-alcohol research, so it should not be presented as cannabis-only evidence; its current recruitment status also needs separate confirmation.
What medical cannabis patients should take away
Some established patients show little change on particular cognitive or simulator measures after their usual dose. Other research demonstrates that THC can impair driving, especially soon after use. Individual responses differ, and a blood THC number by itself cannot quantify the effect. The evidence supports careful distinctions between exposure, measurable performance and current impairment.
Better science. Better testing. Fairer policy.
The test should be how you drive—not simply what remains in your system from Tuesday. Better evidence can inform more accurate testing and safer roads. Follow Mandi Cruz Media for research-based education and explore resources at mandicruzmedia.com.
Research and further reading
Arkell et al., CNS Drugs (2023); Manning et al., Journal of Psychopharmacology (2024); Marcotte et al., JAMA Psychiatry (2022); National Highway Traffic Safety Administration, Marijuana-Impaired Driving: A Report to Congress (2017); Maryland Cannabis Public Health Advisory Council, legislative testimony (January 2026); Johns Hopkins registered study NCT06293040.
Research note: The studies differ in design, participants and outcome measures. Cognitive tests and driving simulators are not interchangeable with real-world crash outcomes. Nothing in these findings establishes that driving after cannabis use is always safe.
Direct research and official-source links
- Arkell et al. (2023), CNS Drugs
- Manning et al. (2024), Journal of Psychopharmacology
- Marcotte et al. (2022), JAMA Psychiatry
- NHTSA, Marijuana-Impaired Driving: A Report to Congress (2017)
- Maryland Cannabis Public Health Advisory Council, January 2026 testimony
- Johns Hopkins University, ClinicalTrials.gov NCT06293040
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