27% of Drunk Drivers in New Zealand Also Tested Positive for Cannabis

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).

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.

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.

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.

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.