Both Alcohol and Cannabis Impaired Vision, but Only Higher Alcohol Doses Worsened Driving

Both alcohol (300 ml and 450 ml wine) and cannabis significantly impaired visual function scores. However, only the higher alcohol dose (450 ml) significantly impaired driving simulator performance. Visual impairment was significantly associated with worse driving performance.

Sixty-four young drivers with alcohol and/or cannabis use history were split into alcohol (n=33) and cannabis (n=31) groups. The alcohol group had two sessions (300 ml and 450 ml red wine). The cannabis group smoked cannabis in one session. Visual function (contrast sensitivity, stereoacuity, intraocular straylight) and driving simulator performance were measured.

Understanding how alcohol and cannabis differently affect driving-relevant abilities helps inform impaired driving policies. The finding that cannabis impairs vision but not necessarily driving performance is nuanced and challenges simple comparisons.

This study adds experimental data to the debate about relative impairment from alcohol versus cannabis. The disconnect between visual impairment and driving performance under cannabis suggests that compensatory mechanisms (like driving more cautiously) may partially offset sensory deficits.

The cannabis group had only one session while the alcohol group had two dose levels, making direct comparison uneven. Cannabis dose was not standardized (participants smoked their own cannabis). The driving simulator may not fully capture real-world driving complexity. Small sample sizes per group.