Uncontrolled Co-Drug Use Limited Attribution in an Older-Driver Cannabis Review

forensic toxicology working group members October 31, 2026 Forensic Toxicology

Four studies in the older-driver review did not control for polydrug use, including one that also lacked alcohol control. Their associations cannot be treated as clean estimates of cannabis alone.

The review identified the adjustment gap

The authors examined seven cross-sectional studies of collision involvement or culpability. In discussing limitations, they specifically noted incomplete control for other substances. This is more concrete than a generic statement that all observational research might have confounding.

Co-exposure and its effects require separate evidence

Other drugs can differ between exposure groups and can affect driving-related outcomes. If they are not accounted for, an association assigned to cannabis may contain contributions from co-exposure. The review discusses possible overestimation, but its available data did not calculate an exact correction for each uncontrolled substance.

The limitation does not determine the answer in one case

It would be equally unwarranted to assume that other drugs explained every observation or that controlling for them would necessarily eliminate a cannabis association. The proper interpretation identifies which comparisons adjusted for which exposures and preserves residual uncertainty. In an individual collision review, actual toxicology, medication history and timing still matter. A meta-analysis cannot supply those missing case facts. The pooled result and its adjustment limitations belong together; neither should be replaced with a claim that one substance was necessarily the sole cause.

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