Prescribed THC and Trail Making: Why Stable Attention Scores Do Not Settle Cognitive Effects

Medicinal cannabis patients can show stable scores on several attention measures while performing worse on a mental-flexibility task. A two-study research program highlights why an expert should identify the particular function measured rather than label a whole battery “normal” or “impaired.”

Two groups and two exposure windows

Stefanidis and colleagues studied 41 prescribed oral-oil users and 37 prescribed flower users in separate samples. Participants completed counterbalanced baseline and post-cannabis appointments approximately one week apart. Testing occurred 90 minutes after oral administration or 15 minutes after vaporization.

Those timings reflect different routes. The groups were not the same people receiving both routes, so their findings do not independently establish that oil and flower have identical effects.

The selective result

After the study’s drug-screen exclusions, several measures of processing speed, sustained/divided attention, and inhibitory control did not significantly change. Trail Making Test part B performance, however, deteriorated after cannabis. The oral study also showed a change in the Trail Making ratio.

These outcomes should remain distinguished. A negative result on one attention test cannot erase a significant finding on another measure. Conversely, one affected task does not prove failure of every cognitive domain.

Why this is not a driving clearance

The battery examined functions relevant to driving, not every aspect of actual road performance. The design used patients’ prescribed products and a no-consumption comparison rather than a uniform product and blinded placebo exposure. Tolerance, symptom relief, dose differences, and residual effects can complicate interpretation.

The research does not establish the full time course of any deficit or a concentration at which an individual is necessarily safe or unsafe. A scheduled assessment is a sampling point, not continuous monitoring.

Questions for expert review

Ask which test changed, how the outcome was scored, which participants were excluded, and how the exposure timing fits the opinion. Compare the individual’s medical condition and use history with the studied population.

The defensible interpretation retains both the selective Trail Making finding and the other null results, without treating either as a complete assessment of an individual’s driving ability.

For related context, see our discussion of selective findings across cannabis memory tests.

Primary research

Kayla B. Stefanidis; Carla Schiemer; Taren Mieran; Andrew Hill; Mark S. Horswill; Mathew J. Summers. 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.