Study Explores Impact of THC on Driving-Related Cognitive Skills in Medical Cannabis Users

A recently published study examined the effects of THC on cognitive functions needed for driving (1). Separated into two studies, researchers in Australia analyzed the effects of both orally ingested oil and vaporized cannabis flower for adults prescribed medical cannabis and compared their neuropsychological performances with and without ingesting cannabis. Tolerance of THC was also considered, as the population in the study had a history of long-term frequent use of cannabis. The study, “Elucidating the acute effects of medically prescribed oral and vaporised delta-9-tetrahydrocannabinol on cognitive functions important for driving,” was published in Drug and Alcohol Review in April 2025, and was funded by the Motor Accident Insurance Commission.

The study was conducted in Queensland, Australia, where driving with THC in an individual’s blood is illegal. For both parts of the study, participants completed baseline assessments without cannabis consumption. Post-cannabis assessments – including information processing speed, sustained and divided attention, inhibitory control, and mental flexibility (1) – were completed a week apart from the baseline.

The first study (orally ingested cannabis) involved 41 adult participants. Neuropsychological tests were completed 90 minutes after one dose of the participants’ medical cannabis prescription. The THC content ranged from 0.3 to 50 mg/mL, and plant origin varied among the prescriptions.

The second study (vaporized cannabis) involved 37 adult participants. Neuropsychological tests were completed 15 minutes after one dose of the participants’ medical cannabis prescription. The THC content of the cannabis used ranged from 17% to 26% (milligrams of THC per gram of product).

Results in both groups included:

• Changes in information processing speed, divided and sustained attention, or inhibitory control performance were not observed, suggesting a role of tolerance in modifying effects in patients
• Significant negative impacts on mental flexibility (the capacity for switching attentional focus) and/or visuomotor attention (the TMT B test) were observed

Some of the limitations of the study noted by researchers included several participants testing positive for THC at the start of each session, even after being requested to refrain from cannabis consumption beforehand, resulting in a possible effect on neurocognitive performance. Differences in doses and health conditions were also noted. Testing at multiple time points was also not completed, which could identify the duration of reduction in performance.

Ultimately, the researchers concluded that tolerance to THC may be present in patients with long-term use. “The question of whether the reduction to TMT B performance translates into functional reductions in driving capacity needs to be addressed in future work,” researchers stated in the study’s conclusion. “Such knowledge will not only contribute to current scientific understanding of the effects of cannabis on cognition and driving but will also serve as a foundational step towards identifying behavioural measures of impairment.”

Reference

1. Stefanidis, KB.; Schiemer, C.; Mieran, T.; Hill, A.; Horswill, MS.; Summers, MJ. Elucidating the acute effects of medically prescribed oral and vaporised delta-9-tetrahydrocannabinol on cognitive functions important for driving. Drug and Alcohol Review. 2025. DOI: 10.1111/dar.14060.