CBD’s Vigilance Findings Were Mixed in a Small College-Student Trial
A small randomized trial found mostly similar performance after CBD and placebo, alongside differences in selected vigilance measures. Neither a blanket claim of impairment nor a blanket claim of cognitive neutrality captures the result.
A single-dose feasibility experiment
Forty healthy college students received 300 mg CBD oil or placebo in a blinded parallel-group study: 21 received CBD and 19 received placebo. Tests were repeated about two hours after administration.
The battery included digit-symbol, trail-making, psychomotor-vigilance and simple-reaction tasks. These assess overlapping but different functions. The study did not directly measure on-road driving.
Faster responses and longer lapses differ
Most between-group comparisons were similar. However, the CBD group showed longer attention-lapse duration on the psychomotor-vigilance task after accounting for baseline performance. One auditory reaction-time comparison favored CBD.
A faster response on one measure cannot cancel a longer lapse on another. The selected differences also do not establish a uniform decline across the entire battery. Early responses must be distinguished from the speed of a valid response.
Practice effects complicate before-and-after claims
Both groups improved on some repeated tests. The authors considered learning effects plausible. Performing better after a dose is not enough to show that the dose improved cognition; the placebo comparison and baseline adjustment matter.
Subjective sleepiness or calmness need not track each performance measure. A person’s report and a sustained-attention task answer different questions.
Why the findings remain preliminary
This small feasibility study used young, selected participants and multiple outcomes. The investigators recognized limited statistical power and the possibility of chance findings.
The experiment did not determine responses in older adults, patients taking interacting medications or repeated users. Selected vigilance signals warrant further study while the many null comparisons remain part of the evidence.
For the separate forensic question, see our discussion of cannabinoid concentrations and evidence of impairment.
Primary research
The effects of cannabidiol on subjective states, cognition, and psychomotor function in healthy adults: A randomized clinical trial. Fundamental & clinical pharmacology (2023). DOI: 10.1111/fcp.12868.
