Behind the Wheel at Work

Cannabis generally refers to the Cannabis sativa plant, or products derived from the plant. Marijuana generally refers to the dried leaves, flowers, stems, and seeds from the plant that contain substantial amounts of delta-9-tetrahydrocannabinol (THC), a psychoactive (that is, mind-altering) compound.¹ THC concentrations in marijuana have increased substantially over the past few decades.² The other well-known naturally occurring compound found in the plant and contained in many products is cannabidiol (CBD), which is not mind-altering.³ Because of its psychoactive effects and the potential for impairment, the THC found in marijuana is the primary concern for driving safety. However, any cannabis product (including CBD products) may contain small amounts of THC.

In the U.S., more states are making it legal to use marijuana for medical and non-medical purposes. However, the U.S. Drug Enforcement Administration (DEA) classifies marijuana as a Schedule I drug, meaning that these drugs have no currently accepted medical use and a high potential for abuse. To date the U.S. Food and Drug Administration (FDA) has not approved the use of cannabis for the treatment of any disease or medical condition.⁴ However, the FDA has approved one cannabis-derived drug product and three synthetic cannabis-related drug products for treatment of specific conditions, including seizures in children, nausea among cancer chemotherapy patients, and anorexia associated with weight loss in AIDS patients. These are available only by prescription from a licensed provider. Common reasons for non-medical marijuana use include users’ desire for short-term euphoria and relaxation.^5-6

Data from the 2019 National Survey on Drug Use and Health (NSDUH) indicates that 44.9 million Americans aged 18 or older used marijuana in the past year. Of those, 24.7 million were employed full-time and 6.8 million part-time. Drug testing results from the Quest Diagnostics Drug Testing Index indicate that overall, 4.4% of the U.S. workforce had a positive urine drug screen in 2018, nearly a 5% increase from 2017. The data also showed a nearly 8% increase in marijuana use among workers from 2017 to 2018, with the number of workers and job applicants who tested positive for marijuana increasing from 2.6% to 2.8%. Positive test results for marijuana urine testing, the most common type of testing done, continue to increase for the general U.S. workforce and for workers in regulated, safety-sensitive industries.* Because marijuana use is increasing across almost all industry sectors, it is likely that an increasing number of workers who drive as part of their job are using the drug.

* Examples of safety-sensitive positions include pilots, drivers (rail, bus, and truck), and workers in nuclear plants.

This depends on several factors, including the person’s previous use of marijuana or other drugs, biology (e.g., genes), gender, how the drug is taken, and how strong the drug is.⁷ Marijuana users may experience altered senses (e.g., seeing brighter colors), altered sense of time, mood changes, impaired body movement, difficulty thinking and problem-solving, memory impairment, hallucinations, delusions, and psychosis. Long-term marijuana use may affect brain function and development in younger users, and it has been linked to mental illness.¹

Driving while impaired by any substance is dangerous. The THC contained in marijuana affects areas of the brain that control the body’s movements, balance, coordination, memory, and judgment.^8-10 It can also impair coordination, distort perception, and lead to memory loss and difficulty in problem-solving.^8-10 Specific to driving, it can slow reaction times and the ability to make decisions.

Meta-analyses research (analysis of combined similar studies) has found that the risk of being involved in a crash significantly increases after marijuana use.^11-16 Still, marijuana’s specific contribution to crash risk is unclear because it can be detected in body fluids for days or even weeks after use and is often combined with alcohol.¹⁴ Several driving simulator and other studies found that many marijuana-impaired drivers tend to drive more cautiously by driving slowly or taking fewer risks, perhaps to compensate for their impairment.¹⁶

Impairment from marijuana use can last several hours and is a significant concern for driving safety, yet determining marijuana impairment is complicated. Impairment from marijuana varies with THC concentration or dose, route of administration, and users’ experience with or tolerance to the drug.¹⁷ Marijuana can be detected through drug testing long after the individual has stopped experiencing any physiological effects and impaired functioning. Also, there isn’t an agreed upon threshold for THC levels to determine impairment. Lastly, there is wide variability in how THC is metabolized by frequent versus infrequent users, which makes interpretation of a positive urine drug test a challenge.¹⁸  For all of these reasons, traditional drug testing (e.g. urine or blood) is not an ideal tool for determining if a driver was impaired at the time of a crash. There is a need for better tools to detect impairment.

Research aimed at better understanding the impacts of marijuana use on workers is needed, including estimates of marijuana use by industry and occupation for those who drive as part of their job. It is also important to take a closer look at how state laws may impact workers’ marijuana use and on-the-job health and safety. Issues related to testing practices and determination of impairment also need to be better understood in order to ensure effective workplace policies and practices.

1. 1. National Institute on Drug Abuse: Marijuana Drug Facts
   2. ElSohly MA, Mehmedic Z, Foster S, Gon C, Chandra S, Church JC. Changes in cannabis potency over the last 2 decades (1995‐2014): analysis of current data in the United States. Biol Psychiatry. 2016; 79(7):613‐619. doi: 10.1016/j.biopsych.2016.01.004
   3. Centers for Disease Control and Prevention: Marijuana and Public Health: What is marijuana?
   4. U.S. Food & Drug Administration: FDA and Cannabis: Research and Drug Approval Process
   5. Howard J, Osborne J. Cannabis and work: Need for more research. Am J Ind Med. 2020;1–10. doi: 10.1002/ajim.23170
   6. Whiting PF, Wolff RF, Deshpande S, et al. Cannabinoids for medical use: a systematic review and meta‐analysis. JAMA. 2015;313(24): 2456‐2773. doi: 10.1001/jama.2015.6358
   7. Centers for Disease Control and Prevention: Marijuana and Public Health: What determines how marijuana effects a person?
   8. Lenné MG, Dietze PM, Triggs TJ, Walmsley S, Murphy B, Redman JR. The effects of cannabis and alcohol on simulated arterial driving: Influences of driving experience and task demand. Accid Anal Prev. 2010;42(3):859-866. doi: 10.1016/j.aap. 2009.04.021.
   9. Hartman RL, Huestis MA. Cannabis effects on driving skills. Clin Chem. 2013;59(3):478-492. doi: 10.1373/clinchem.2012.194381.
   10. Hartman RL, Brown TL, Milavetz G, et al. Cannabis effects on driving lateral control with and without alcohol. Drug Alcohol Depend. 2015;154:25-37. doi: 10.1016/j. drugalcdep.2015.06.015.
   11. Elvik R. Risk of road accident associated with the use of drugs: a systematic review and meta-analysis of evidence from epidemiological studies. Accid Anal Prev. 2013;60:254-267. doi: 10.1016/j.aap.2012.06.017.
   12. Ramaekers JG, Berghaus G, van Laar M, Drummer OH. Dose related risk of motor vehicle crashes after cannabis use. Drug Alcohol Depend. 2004;73(2):109-119. doi: 10.1016/j.drugalcdep.2003.10.008.
   13. Li M-C, Brady JE, DiMaggio CJ, Lusardi AR, Tzong KY, Li G. Marijuana Use and Motor Vehicle Crashes. Epidemiol Rev. 2012;34(1):65-72. doi: 10.1093/epirev/mxr017.
   14. 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. doi: 10.1136/bmj.e536.
   15. Rogeberg O, Elvik R. The effects of cannabis intoxication on motor vehicle collision revisited and revised. Addiction. 2016 Aug;111(8):1348-59. doi: 10.1111/add.13347. Epub 2016 Apr 25. Erratum in: Addiction. 2018 May;113(5):967-969. PMID: 26878835
   16. Compton, R. (2017, July). Marijuana-Impaired Driving – A Report to Congress. (DOT HS 812 440). Washington, DC: National Highway Traffic Safety Administration.
   17. Phillips, J. A., Holland, M. G., Baldwin, D. D., Gifford-Meuleveld, L., Mueller, K. L., Perkison, B., … Dreger, M. (2015). Marijuana in the Workplace: Guidance for Occupational Health Professionals and Employers: Joint Guidance Statement of the American Association of Occupational Health Nurses and the American College of Occupational and Environmental Medicine. Workplace Health & Safety, 63(4), 139–164. doi: 10.1177/2165079915581983.
   18. NIDA News Release: Research on THC blood levels sheds light on difficulties of testing for impaired driving.