Cannabis Impairment Is Not the Same as Recent Cannabis Use

A biological result may indicate that cannabis was consumed recently, but it does not automatically describe how safely a person can drive or work. Impairment can vary with dose, product, tolerance, time since use and the individual. Researchers are therefore testing combinations of behavioural, physiological and biological measurements.

The observational study included 125 adults. Participants who used cannabis inhaled a self-selected product over 15 minutes, while non-users served as controls. Researchers collected a tablet-based psychomotor test, handheld pupil measurements and blood cannabinoid data before use and at approximately 40 and 100 minutes after inhalation began.

The researchers used a driving simulator and defined impairment as a change of more than six centimetres in the standard deviation of lateral position, a measure related to lane control. They then used penalized logistic-regression models to predict either recent cannabis inhalation or the simulator-based impairment outcome.

Models that combined information from all data sources performed better than models using a single source. At 40 minutes, the combined recent-use model had an area under the curve of 0.996, while the impairment model reached 0.886. This difference shows that recent inhalation was easier to classify than the selected impairment outcome.

At 100 minutes, performance declined for both outcomes. The recent-use model still performed strongly, but the impairment model was weaker than at 40 minutes. That pattern reinforces the challenge: drug detection and functional impairment change over time and do not follow one simple, universal relationship.

This was a research study, not a validated roadside or workplace test. Participants used different self-selected products, the sample was limited, and impairment was defined using one simulator threshold. Larger studies must test different products, routes, use histories, real-world settings and longer time periods before operational use is considered.

Difficulty measuring impairment does not make driving after cannabis safe. Cannabis can affect attention, reaction time, coordination and judgment. Plan transportation before consuming, use a sober driver or other safe option, and never use a detection result or personal feeling as proof that it is safe to drive.

Research and source notes: Bird SM et al. Journal of Safety Research. 2026;97:142-151. doi:10.1016/j.jsr.2026.02.004 • Health Canada: Cannabis impairment and driving