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

The takeaway

In an experimental comparison, both alcohol and cannabis impaired visual function, but only the higher alcohol dose significantly worsened driving simulator performance, and visual impairment was linked to worse driving.

Traffic safety researchers; policymakers setting impaired driving thresholds; anyone interested in the comparative effects of alcohol versus cannabis on driving.

Cannabis impaired vision but did not significantly worsen driving performance

What the researchers found

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.

Why it matters

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.

The numbers in context

64 young drivers tested. Visual impairment (OVS) was significant for all conditions: alcohol 300 ml (p=0.005), alcohol 450 ml (p<0.001), and cannabis (p=0.028). Driving performance was only significantly impaired at the higher alcohol dose. Visual impairment was significantly associated with worse driving.

How the study worked

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.

What this study cannot tell us

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.

How to read the evidence

Moderate: controlled experimental design with objective visual and driving measures, but limited by unequal dosing conditions and moderate sample size.

When this study was published

Published in 2022.

The bigger picture

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.

Questions still open

Would higher cannabis doses produce driving impairment comparable to alcohol? Do cannabis users compensate for visual impairment by driving more cautiously? How should impaired driving laws account for different impairment profiles between substances?

Common questions

Is it safe to drive after using cannabis?

This study found cannabis impaired visual function even though it did not significantly affect driving simulator scores. Visual impairment was linked to worse driving overall, so the results do not support driving after cannabis use.

Is cannabis less impairing than alcohol for driving?

In this study, the higher alcohol dose impaired both vision and driving performance, while cannabis impaired vision without significantly affecting the driving score. However, this does not mean cannabis is "safe" for driving. The comparison was limited by unequal dosing protocols.

Read the original research

Comparison of the effects of alcohol and cannabis on visual function and driving performance. Does the visual impairment affect driving?

Drug and alcohol dependence, 237, 109538

View on PubMed ↗Publisher’s paper ↗

Citation

Ortiz-Peregrina, Sonia; Casares-López, Miriam; Ortiz, Carolina; Castro-Torres, José J; Martino, Francesco; Jiménez, José R. (2022). Comparison of the effects of alcohol and cannabis on visual function and driving performance. Does the visual impairment affect driving?. Drug and alcohol dependence, 237, 109538. https://doi.org/10.1016/j.drugalcdep.2022.109538