The Hazard-Perception Study Did Not Measure Blood THC Concentration-Performance Relationships

The medicinal-oil hazard-perception experiment did not collect blood measures that could quantify THC and metabolite concentrations for each condition. Its task findings therefore could not validate a blood concentration-performance relationship.

The investigators identified the missing measurement

The paper explicitly notes that absent blood measures prevented quantification of THC and relevant metabolites and examination of their association with task performance. Oral-fluid screening supplied a different type of information and did not fill this gap.

A prescribed amount is not a concentration

Participants used oils with variable THC doses, strain characteristics and CBD content. A nominal administered amount cannot be substituted for a measured blood concentration. Nor can a screen indication supply the continuous exposure data needed to estimate a concentration-response curve.

Do not borrow a threshold from a task null

The absence of a significant hazard-perception decline does not identify a blood value at which drivers are necessarily unimpaired. The study did not test that proposition. Equally, the missing measurement does not prove concentrations were high, low or unrelated to the outcome. Its contribution concerns video-based performance under the observed use conditions. A forensic concentration opinion needs suitable specimen data and evidence connecting the matrix, sampling time and functional question rather than imputing those measurements to this study.

Related analysis: the study’s main findings and limitations.

Research cited

Mieran and colleagues. The effects of orally ingested Delta-9-Tetrahydrocannabinol on drivers’ hazard perception and risk-taking behaviours: A within-subjects study of medicinal cannabis users. Psychopharmacology. DOI: 10.1007/s00213-025-06869-w.