Among 3,050 New Zealand drivers caught over the legal blood alcohol limit, 27% had also used cannabis and 16% had used prescription medications.
Traffic safety researchers, law enforcement, policymakers designing roadside drug testing programs.
27% of drunk drivers also positive for cannabis
What the researchers found
Among 3,050 drivers with blood alcohol exceeding legal limits in New Zealand (2011-2015), 41% had used alcohol in combination with other drugs. Cannabis was the most common co-intoxicant at 27% (816 drivers), followed by prescription medications at 16% (500 drivers) and other illicit drugs at 2.7%. Top prescription drugs combined with alcohol were antidepressants (citalopram, fluoxetine, venlafaxine), an antipsychotic (quetiapine), a sedative (diazepam), and an opioid (tramadol).
Why it matters
This study reveals that drunk driving rarely involves alcohol alone. More than one in four drunk drivers also had cannabis in their system, and one in six had prescription medications. The high rate of poly-substance use among impaired drivers has implications for both roadside testing strategies and understanding crash risk.
The numbers in context
3,050 drivers tested. 41% (1,235) had alcohol plus drugs. 27% (816) had cannabis. 16% (500) had prescription meds. 2.7% (81) had other illicit drugs. Drug use did not correlate with alcohol amount. Top prescriptions: citalopram, fluoxetine, venlafaxine, quetiapine, diazepam, tramadol.
How the study worked
Blood samples from 3,050 drivers who failed breath alcohol tests and elected laboratory blood analysis during 2011-2015 were tested for alcohol and a range of drugs using LC-TOFMS and cannabis immunoassay. Drug use was analyzed for prevalence, co-use patterns, and correlation with alcohol levels.
What this study cannot tell us
Biased sample: only includes drivers who failed breath tests, elected blood tests, and exceeded legal limits. Cannot generalize to all drivers. Detection of cannabis does not confirm impairment at time of driving. Prescription medications may have been taken as prescribed. Study period (2011-2015) may not reflect current patterns.
How to read the evidence
Moderate: large sample with laboratory-confirmed drug testing, though biased toward drivers who failed breath tests and cannot assess impairment.
When this study was published
2025 study (data from 2011-2015)
The bigger picture
Roadside enforcement typically focuses on alcohol detection, but this data suggests that cannabis and prescription drug impairment are widespread among the same drivers. Multiple substance use could compound impairment beyond what either substance would cause alone, though this study did not assess driving performance.
Questions still open
- Does combining cannabis with alcohol increase crash risk beyond either substance alone? Are prescription medication users aware of impairment risks when combined with alcohol? Would expanded roadside drug testing change impaired driving patterns?
Common questions
How common is it for drunk drivers to also be using cannabis?
Does this mean cannabis makes driving worse when combined with alcohol?
Read the original research
The use of prescription medication and other drugs by New Zealand drivers with illegal blood alcohol levels.
Traffic injury prevention, 26(4), 389-397
Citation
Poulsen, Helen; Raymond, Onyekachi; McCarthy, Mary Jane. (2025). The use of prescription medication and other drugs by New Zealand drivers with illegal blood alcohol levels.. Traffic injury prevention, 26(4), 389-397. https://doi.org/10.1080/15389588.2024.2418361