“Drug driving” refers to driving while using impairing drugs. This includes illegal drugs such as cocaine, amphetamines, and heroin. It includes cannabis, which is illegal for recreational use but can be used legally for medical purposes. Drug driving also refers to driving while using any of several hundred prescription or over-the-counter medications.

Dealing with drug driving is complicated. There are hundreds of different drugs with different effects, half-lives, and routes of ingestion, which makes identifying drugs in drivers difficult to detect. Although this situation limits our understanding of drug driving, we do know some facts.

Many BC drivers use drugs
Researchers have conducted regular roadside surveys for drug use in BC since 1995. Drivers are randomly selected and asked to provide breath and saliva samples, which are analyzed for alcohol and drugs. In the latest survey (2012), 8.3% of drivers had been drinking and 10.1% had used at least one drug. Cannabis was found in 5.4% of drivers, cocaine in 4.1%, amphetamines in 1.7%, opiates in 1.1%, and benzodiazepines in 0.1%.[1] Additional evidence comes from an ongoing study of injured drivers treated in BC trauma centres. The results from the first 1097 drivers from this study show the following: alcohol was present in 17.8% of drivers, and the alcohol concentration was greater than 0.08 in 15.4%. The most common recreational drug was cannabis: 12.6% of drivers tested positive for cannabis metabolites, and 7.3% were positive for THC, indicating recent use. Cannabis was more common in males and in drivers younger than 30 years of age. Cocaine was positive in 2.8% of drivers and amphetamines in 1.2%. Medications including diphenhydramine (11%), benzodiazepines (5.0%), zopiclone (0.6%), antidepressants (6.5%), and opiates (4.9%) were also detected. In total, 26.1% of drivers were positive for at least one sedating medication. Drivers older than 50 years of age were more likely to use medications.[2] 

Many drugs impair skills required for safe driving
Cannabis slows reaction times, causes weaving, creates difficulty maintaining a constant speed, and predisposes to distraction.[3] Cocaine and amphetamines are CNS stimulants that increase wakefulness and may improve reaction time. They also impair judgment, may be associated with aggressive and risky driving, and are associated with rebound sleepiness following a binge. 

Many prescription medications also cause impairment. Opiates, such as codeine or hydromorphone, cause drowsiness, which can be extreme in naive users. Opiates also impair coordination, slow reaction time, and decrease ability to concentrate. Numerous medications are CNS depressants. These include benzodiazepines, Z-drugs (such as zopiclone), antidepressants, antihistamines, and many others. CNS depressants impair coordination, cause drowsiness, and reduce attention to the task of driving. Unfortunately, many drivers take drugs together with alcohol, resulting in even greater impairment.[4,5] 

Limited real-world evidence linking drugs to crashes
Drugs cause impairment but, if drivers are aware of their impairment, they may be able to compensate by driving more slowly or avoiding complex driving situations or risky maneuvers. In addition, chronic users often develop a tolerance to the impairing effects of medications. Therefore, real-world evidence is required to quantify the risk of crashing associated with drug use.

Cannabis is the most-studied drug. Evidence suggests that acute cannabis use approximately doubles the risk of crashing.[6] For stimulants (cocaine, amphetamines), some studies suggest an increase crash risk, but evidence is limited.[7,8] Prescription medications are difficult to study because of the confounding effect of the underlying illness, the necessity of controlling for tolerance, and the fact that many people use multiple psychotropic medications. Benzodiazepines appear to increase crash risk by 40% to 60%.[9,10] Unfortunately, evidence is lacking for other prescription medications.[11]

Further research is urgently needed to better understand the crash risk associated with prescription medications.

Drug-impaired drivers are difficult for police to detect and enforcement of current drug driving laws is difficult.

Doctors of BC encourages the governments of BC and Canada to promote and provide funding for the development of improved screening tools to detect drug-impaired drivers and to work with stakeholders to improve legislation and policies to address the problem of drug-impaired driving.
—Jeff Brubacher, MD
Emergency Medical Services Committee

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This article is the opinion of the Council on Health Promotion and has not been peer reviewed by the BCMJ Editorial Board.

References

1.    Beasley EE, Beirness DJ. Alcohol and drug use among drivers following the introduction of immediate roadside prohibitions in British Columbia: Findings from the 2012 roadside survey. Accessed 27 February 2015. www.pssg.gov.bc.ca/osmv/data/index.htm [10].
2.    Brubacher JR, Chan H, Martz W, et al. Recent drug use in injured British Columbia drivers. Presented at the Canadian Multidisciplinary Road Safety Conference, Vancouver, BC, 12 July 2014.
3.    Ramaekers JG, Berghaus G, van Laar M, et al. Dose related risk of motor vehicle crashes after cannabis use. Drug Alcohol Depend 2004;73:109-119.
4.    Maxwell HG, Dubois S, Weaver B, et al. The additive effects of alcohol and benzodiazepines on driving. Can J Public Health 2010;101:353-357.
5.    Hartman RL, Huestis MA. Cannabis effects on driving skills. Clin Chem 2013;59:478-492.
6.    Asbridge M, Hayden JA, Cartwright JL. Acute cannabis consumption and motor vehicle collision risk: Systematic review of observational studies and meta-analysis. BMJ 2012;344:e536.
7.    Drummer OH, Gerostamoulos J, Batziris H, et al. The involvement of drugs in drivers of motor vehicles killed in Australian road traffic crashes. Accid Anal Prev 2004;36:239-248.
8.    Laumon B, Gadegbeku B, Martin JL, et al. Cannabis intoxication and fatal road crashes in France: Population based case-control study. BMJ 2005;331:1371.
9.    Rapoport MJ, Lanctot KL, Streiner DL, et al. Benzodiazepine use and driving: A meta-analysis. J Clin Psychiatry 2009;70:663-673.
10.    Orriols L, Philip P, Moore N, et al. Benzodiazepine-like hypnotics and the associated risk of road traffic accidents. Clin Pharmacol Ther 2011;89:595-601.
11.    Orriols L, Salmi LR, Philip P, et al. The impact of medicinal drugs on traffic safety: A systematic review of epidemiological studies. Pharmacoepidemiol Drug Saf 2009;18:647-658.