This is a list of studies detailing some of the negative effects of marijuana. The list is divided into five categories: (1) effects of marijuana on cognition, (2) effects of legalization on use, (3) effects of legalization on hospitalization, (4) the connection between marijuana and other drugs, and (5) the effects of marijuana on motor vehicle accidents. All studies are recent.

Last updated: 6/12/2019

Effects of Marijuana on Cognition

1. “There is substantial evidence of a statistical association between cannabis use and the development of schizophrenia or other psychoses, with the highest risk among the most frequent users.”

“There is moderate evidence of a statistical association between cannabis use and:

• Increased symptoms of mania and hypomania in individuals diagnosed with bipolar disorders (regular cannabis use)
• A small increased risk for the development of depressive disorders
• Increased incidence of suicidal ideation and suicide attempts with a higher incidence among heavier users
• Increased incidence of suicide completion
• Increased incidence of social anxiety disorder (regular cannabis use)”
  National Academies of Sciences, Engineering, and Medicine. 2017. The Health Effects of Cannabis and Cannabinoids: The Current State of Evidence and Recommendations for Research. Washington, DC: The National Academies Press. https://doi.org/10.17226/24625.

2. “The preponderance of evidence… substantiates not only a significant, causal role for marijuana in chronic psychotic syndromes but also a strong association with mood disorders and suicidal ideation. Thus, there can no longer be any doubt that the range of negative mental health impacts of this drug, too frequently dismissed as fear-mongering rhetoric, must be positioned at the front and center of international drug policy dialogue.”
   Miller, C. L. (2018). “The Impact of Marijuana on Mental Health” in K. Winters and K. Sabet, Contemporary Health Issues in Marijuana. Oxford: Oxford University Press. doi: 10.1093/med-psych/9780190263072.003.0006
3. “Marijuana use has been associated with substantial adverse effects, some of which have been determined with a high level of confidence. Marijuana, like other drugs of abuse, can result in addiction. During intoxication, marijuana can interfere with cognitive function (e.g., memory and perception of time) and motor function (e.g., coordination), and these effects can have detrimental consequences (e.g., motor-vehicle accidents). Repeated marijuana use during adolescence may result in long-lasting changes in brain function that can jeopardize educational, professional, and social achievements.”
   Volkow, N. D., Baler, R. D., Compton, W. M., & Weiss, S. R. B. (2014). Adverse Health Effects of Marijuana Use. New England Journal of Medicine, 370(23), 2219–2227. doi:10.1056/nejmra1402309
4. “The epidemiological literature in the past 20 years shows that cannabis use increases the risk of accidents and can produce dependence, and that there are consistent associations between regular cannabis use and poor psychosocial outcomes and mental health in adulthood.”
   Hall, W. (2014). What has research over the past two decades revealed about the adverse health effects of recreational cannabis use? Addiction, 110(1), 19–35. doi:10.1111/add.12703
5. “A growing body of evidence indicates that relative to non-MJ users, heavy MJ users exhibit poorer performance on cognitive tasks, altered patterns of brain activity, and lower frontal WM coherence, which are highly moderated by age of onset of MJ use.”
   Gruber, S. A., & Sagar, K. A. (2017). Marijuana on the Mind? The Impact of Marijuana on Cognition, Brain Structure, and Brain Function, and Related Public Policy Implications. Policy Insights from the Behavioral and Brain Sciences, 4(1), 104–111. doi:10.1177/2372732216684851
6. “Cannabis use results in an acute impairment of executive function, with the strongest effects on inhibitory control.”
   Oomen, P. P., van Hell, H. H., & Bossong, M. G. (2018). The acute effects of cannabis on human executive function. Behavioural Pharmacology, 1. doi:10.1097/fbp.0000000000000426
7. “There is… robust evidence to suggest that cannabis use… confer[s] a higher risk for developing schizophrenia spectrum disorders.”
   Belbasis, L., Köhler, C. A., Stefanis, N., Stubbs, B., van Os, J., Vieta, E., … Evangelou, E. (2017). Risk factors and peripheral biomarkers for schizophrenia spectrum disorders: an umbrella review of meta-analyses. Acta Psychiatrica Scandinavica, 137(2), 88–97. doi:10.1111/acps.12847
8. “The literature… provide strong evidence that chronic cannabis abuse causes cognitive impairment and damages the brain, particularly white matter, where cannabinoid 1 receptors abound.”
   Mandelbaum, D. E., & de la Monte, S. M. (2017). Adverse Structural and Functional Effects of Marijuana on the Brain: Evidence Reviewed. Pediatric Neurology, 66, 12–20. doi:10.1016/j.pediatrneurol.2016.09.004
9. “It is well established that cannabis use causes acute impairment in the ability of the brain to hold information (ie, cognitive capacity). Hence, temporary deficits occur in learning and memory, attention, and working memory.”
   Volkow, N. D., Swanson, J. M., Evins, A. E., DeLisi, L. E., Meier, M. H., Gonzalez, R., … Baler, R. (2016). Effects of Cannabis Use on Human Behavior, Including Cognition, Motivation, and Psychosis: A Review. JAMA Psychiatry, 73(3), 292. doi:10.1001/jamapsychiatry.2015.3278
10. “Verbal learning and memory, and attention, continue to be most consistently impaired by acute and chronic exposure to cannabis. Psychomotor function is most affected during the acute intoxication, with some evidence for persistence in chronic users and following cessation of use.”
    Broyd, S. J., van Hell, H. H., Beale, C., Yücel, M., & Solowij, N. (2016). Acute and Chronic Effects of Cannabinoids on Human Cognition—A Systematic Review. Biological Psychiatry, 79(7), 557–567. doi:10.1016/j.biopsych.2015.12.002
11. “The 68 identified reviews reported harm for 62 outcomes, insufficient evidence of harm for 20 outcomes and no evidence of harm for 6 outcomes… [T]he general conclusion is that marijuana is associated with negative effects on several aspects of mental and physical health.”
    CMAJ Open. 2018 Jul-Sep; 6(3): E339–E346.
12. “Overall, evidence from epidemiologic studies provides strong enough evidence to warrant a public health message that cannabis use can increase the risk of psychotic disorders.
    Gage, S. H., Hickman, M., & Zammit, S. (2016). Association Between Cannabis and Psychosis: Epidemiologic Evidence. Biological Psychiatry, 79(7), 549–556. doi:10.1016/j.biopsych.2015.08.001
13. “Individuals with an early history of heavy use of cannabis are at a higher risk of death than those with a history of no use of cannabis.”
    Manrique-Garcia, E., Ponce de Leon, A., Dalman, C., Andréasson, S., & Allebeck, P. (2016). Cannabis, Psychosis, and Mortality: A Cohort Study of 50,373 Swedish Men. American Journal of Psychiatry, 173(8), 790–798. doi:10.1176/appi.ajp.2016.14050637
14. “Current evidence shows that high levels of cannabis use increase the risk of psychotic outcomes and confirms a dose-response relationship between the level of use and the risk for psychosis.”
    Marconi, A., Di Forti, M., Lewis, C. M., Murray, R. M., & Vassos, E. (2016). Meta-analysis of the Association Between the Level of Cannabis Use and Risk of Psychosis. Schizophrenia Bulletin, 42(5), 1262–1269. doi:10.1093/schbul/sbw003
15. “It is now incontrovertible that heavy use of cannabis increases the risk of psychosis.”
    Colizzi, M., & Murray, R. (2018). Cannabis and psychosis: what do we know and what should we do? The British Journal of Psychiatry, 212(4), 195–196. doi:10.1192/bjp.2018.1
16. “[N]europsychological studies provide evidence for subtle cognitive deficits at least 7 days after heavy cannabis use. The structural neuroimaging studies show growing evidence of abnormalities in hippocampus volume and gray matter density of cannabis users relative to controls.”
    Nader, D. A., & Sanchez, Z. M. (2017). Effects of regular cannabis use on neurocognition, brain structure, and function: a systematic review of findings in adults. The American Journal of Drug and Alcohol Abuse, 44(1), 4–18. doi:10.1080/00952990.2017.1306746
17. “There is an elevated risk of experiencing psychotic-like symptoms even among those with minimal lifetime exposure to cannabis. Cognitive deficits after acute exposure and in chronic users are most often reported in the domains of verbal learning and memory; working memory; processing speed, attention, and psychomotor function; and aspects of executive function.”
    Solowij, N. (2018). Psychotomimetic and Cognitive Effects of Cannabis Use in the General Population. The Complex Connection Between Cannabis and Schizophrenia, 129–155. doi:10.1016/b978-0-12-804791-0.00005-7
18. “An extensive growing body of research has demonstrated that cannabis use affects cognitive performance adversely. In particular, deficits were found in working and episodic memory, as well as in executive and attentional functions. Although some studies reported recovery with prolonged abstinence, others reported persisting impairments in several cognitive domains, such as, attention, working memory, and executive functioning.”
    Evren, C. (2017). “Cannabis Use and Cogntive Function” in Handbook of Cannabis and Related Pathologies (Academic Press, 2017): 70-78, doi: 10.1016/B978-0-12-800756-3.00009-0
    

Legalization Increases Use

1. “We find increases in the probability of current marijuana use, regular marijuana use and marijuana abuse/dependence among those aged 21 or above.”
   Wen, H., Hockenberry, J. M., & Cummings, J. R. (2015). The effect of medical marijuana laws on adolescent and adult use of marijuana, alcohol, and other substances. Journal of Health Economics, 42, 64–80. doi:10.1016/j.jhealeco.2015.03.007
2. “Medical marijuana laws appear to have contributed to increased prevalence of illicit cannabis use and cannabis use disorders.”
   Hasin, D. S., Sarvet, A. L., Cerdá, M., Keyes, K. M., Stohl, M., Galea, S., & Wall, M. M. (2017). US Adult Illicit Cannabis Use, Cannabis Use Disorder, and Medical Marijuana Laws. JAMA Psychiatry, 74(6), 579. doi:10.1001/jamapsychiatry.2017.0724
3. “MU increased after passage of MML among adults ages 26–39, 40–64, and 65+. These increases were partially mediated by PA of marijuana.”
   Mauro, C., Santaella, J., Kim, J. H., Wall, M. M., & Martins, S. S. (2017). Does perceived availability of marijuana explain changes in marijuana use after medical marijuana law implementation among U.S. adults? Drug and Alcohol Dependence, 171, e134. doi:10.1016/j.drugalcdep.2016.08.371
4. “Using survey data from the National College Health Assessment, we show that students at Washington State University experienced a significant increase in marijuana use after legalization. This increase is larger than would be predicted by national trends. The change is strongest among females, Black students, and Hispanic students. The increase for underage students is as much as for legal-age students.”
   Miller, A. M., Rosenman, R., & Cowan, B. W. (2017). Recreational marijuana legalization and college student use: Early evidence. SSM – Population Health, 3, 649–657. doi:10.1016/j.ssmph.2017.08.001
5. “After controlling for the historically declining trend in marijuana use prior to the passages of MML in Michigan, we found that marijuana use among adolescents had increased subsequent to the passage of state MML.”
   Chen, X., Yu, B., Stanton, B., Cook, R. L., Chen, D.-G., & Okafor, C. (2018). Medical Marijuana Laws and Marijuana Use Among U.S. Adolescents: Evidence From Michigan Youth Risk Behavior Surveillance Data. Journal of Drug Education, 004723791880336. doi:10.1177/0047237918803361
6. “Among adolescents, after controlling for selection biases, adolescents who lived in a state that legalized medical marijuana were more likely to use marijuana (OR=1.74, p<.001), compared to those who lived in a state that medical marijuana was illegal. Among adults, those who lived in a state that medical marijuana was legalized were also more likely to use marijuana (OR=1.59, p<.001), compared to those who lived in a state that medical marijuana was illegal.”
   Wong, S., & Lin, H. (2016). Effects of Medical Marijuana Legalization on Marijuana Use Among Adolescents and Adults In The United States. Value in Health, 19(7), A527–A528. doi:10.1016/j.jval.2016.09.1053
7. “Results indicated that cannabis use increased since recreational legalization for all students, but more so for those over 21 years.”
   Parnes, J. E., Smith, J. K., & Conner, B. T. (2018). Reefer madness or much ado about nothing? Cannabis legalization outcomes among young adults in the United States. International Journal of Drug Policy, 56, 116–120. doi:10.1016/j.drugpo.2018.03.011
8. “Among eighth and 10th graders in Washington, perceived harmfulness of marijuana use decreased and marijuana use increased following legalization of recreational marijuana use.”
   Cerdá, M., Wall, M., Feng, T., Keyes, K. M., Sarvet, A., Schulenberg, J., … Hasin, D. S. (2017). Association of State Recreational Marijuana Laws With Adolescent Marijuana Use. JAMA Pediatrics, 171(2), 142. doi:10.1001/jamapediatrics.2016.3624
9. “After controlling for the historically declining trend in marijuana use prior to the passages of MML in Michigan, we found that marijuana use among adolescents had increased subsequent to the passage of state MML.”
   CChen, X., Yu, B., Stanton, B., Cook, R. L., Chen, D.-G., & Okafor, C. (2018). Medical Marijuana Laws and Marijuana Use Among U.S. Adolescents: Evidence From Michigan Youth Risk Behavior Surveillance Data. Journal of Drug Education, 004723791880336. doi:10.1177/00472379188033614
10. “Cannabis liberalization with depenalization and partial prohibition policies was associated with higher levels of regular cannabis use among adolescents.”
    Shi, Y., Lenzi, M., & An, R. (2015). Cannabis Liberalization and Adolescent Cannabis Use: A Cross-National Study in 38 Countries. PLOS ONE, 10(11), e0143562. doi:10.1371/journal.pone.0143562

Hospitalizations

1. “[D]ata demonstrate a significant increase in adolescent marijuana-associated emergency department and urgent cares visits in Colorado.”
   Wang, G. S., Davies, S. D., Halmo, L. S., Sass, A., & Mistry, R. D. (2018). Impact of Marijuana Legalization in Colorado on Adolescent Emergency and Urgent Care Visits. Journal of Adolescent Health, 63(2), 239–241. doi:10.1016/j.jadohealth.2017.12.010
2. “Despite multiple public health interventions in legislation after 2014, the incidence of CH [children’s hospital] visits and RPC [regional poison center] calls has continued to rise in Colorado with an observed doubling of CH visits in 2017. The public increase in marijuana availability due to legalization likely translates into increased accessibility in the home for children.”
   Wang, G. S., Hoyte, C., Roosevelt, G., & Heard, K. (2018). The Continued Impact of Marijuana Legalization on Unintentional Pediatric Exposures in Colorado. Clinical Pediatrics, 000992281880520. doi:10.1177/0009922818805206
3. “Colorado RPC cases for pediatric marijuana increased significantly and at a higher rate than the rest of the United States. The number of children’s hospital visits and RPC case rates for marijuana exposures increased between the 2 years prior to and the 2 years after legalization. Almost half of the patients seen in the children’s hospital in the 2 years after legalization had exposures from recreational marijuana, suggesting that legalization did affect the incidence of exposures.”
   Wang, G. S., Le Lait, M.-C., Deakyne, S. J., Bronstein, A. C., Bajaj, L., & Roosevelt, G. (2016). Unintentional Pediatric Exposures to Marijuana in Colorado, 2009-2015. JAMA Pediatrics, 170(9), e160971. doi:10.1001/jamapediatrics.2016.0971
4. “Hospital discharges coded as marijuana-dependent increased 1% per month (95% CI=0.8, 1.1, p<0.001) from 2007 to 2013. A change in trend was detected in poison center calls mentioning marijuana (p<0.01). After 2009, poison center calls increased 0.8% per month (95% CI=0.2, 1.4, p<0.01). Poison center calls also increased 56% (95% CI=49%, 63%, p<0.001) in the period following the policy change. Further, there was one hospital discharge coded as dependent for every 3,159 (95% CI=2465, 3853, p<0.001) medical marijuana registrant applications.”
   Davis, J. M., Mendelson, B., Berkes, J. J., Suleta, K., Corsi, K. F., & Booth, R. E. (2016). Public Health Effects of Medical Marijuana Legalization in Colorado. American Journal of Preventive Medicine, 50(3), 373–379. doi:10.1016/j.amepre.2015.06.034
5. “From 2000 to 2015, hospitalization rates with marijuana-related billing codes increased from 274 to 593 per 100,000 hospitalizations in 2015. Overall, the prevalence of mental illness among ED visits with marijuana-related codes was five-fold higher (5.07, 95% CI: 5.0, 5.1) than the prevalence of mental illness without marijuana-related codes. RPC calls remained constant from 2000 through 2009. However, in 2010, after local medical marijuana policy liberalization, the number of marijuana exposure calls significantly increased from 42 to 93; in 2014, after recreational legalization, calls significantly increased by 79.7%, from 123 to 221 (p < 0.0001). The age group < 17 years old also had an increase in calls after 2014.”
   Wang, G. S., Hall, K., Vigil, D., Banerji, S., Monte, A., & VanDyke, M. (2017). Marijuana and acute health care contacts in Colorado. Preventive Medicine, 104, 24–30. doi:10.1016/j.ypmed.2017.03.022
6. “The average number (per 1000 ED visits) of marijuana exposure visits to the Emergency Department increased from 4.3 to 6.4 before and after legalization. Also, there was an average increase in poison control cases for exposures to marijuana by 34% each year in Colorado as compared to an increase by 19% for the remainder of the country.”
   Diebold, S. (2017). Unintentional Pediatric Exposures to Marijuana in Colorado, 2009-1015. The Journal of Emergency Medicine, 52(3), 390. doi:10.1016/j.jemermed.2017.01.034
7. “[H]ealthcare encounters with mention of marijuana have increased after legalization. Poison center calls among 9–17-year-olds have markedly increased, while calls for 18–25-year-olds saw a modest increase. Meanwhile, the portion of hospitalizations with a marijuana code increased modestly among 9–17-year-olds but tripled among 18–25-year-olds, reaching 8.1% in 2014–2015.
   Vigil, D. I., Van Dyke, M., Hall, K. E., Contreras, A. E., Ghosh, T. S., & Wolk, L. (2018). Marijuana Use and Related Health Care Encounters in Colorado Before and After Retail Legalization. International Journal of Mental Health and Addiction, 16(4), 806–812. doi:10.1007/s11469-018-9901-0
   
8. “Despite multiple preventative measures, pediatric unintentional marijuana exposures continue to increase 4 years after legalization of recreational use in Colorado.”
   Wang, G. S., Hoyte, C., Roosevelt, G., & Heard, K. (2018). The Continued Impact of Marijuana Legalization on Unintentional Pediatric Exposures in Colorado. Clinical Pediatrics, 000992281880520. doi:10.1177/0009922818805206

Opioids and Other Drugs

1. “Our findings disconfirm the hypothesis that a population-level negative correlation between medical marijuana use and prescription drug harms occurs because medical marijuana users are less likely to use prescription drugs, either medically or nonmedically. Medical marijuana users should be a target population in efforts to combat nonmedical prescription drug use.”
   Caputi, T. L., & Humphreys, K. (2018). Medical Marijuana Users are More Likely to Use Prescription Drugs Medically and Nonmedically. Journal of Addiction Medicine, 12(4), 295–299. doi:10.1097/adm.0000000000000405
2. “Cannabis use appears to increase rather than decrease the risk of developing nonmedical prescription opioid use and opioid use disorder.”
   Olfson, M., Wall, M. M., Liu, S.-M., & Blanco, C. (2018). Cannabis Use and Risk of Prescription Opioid Use Disorder in the United States. American Journal of Psychiatry, 175(1), 47–53. doi:10.1176/appi.ajp.2017.17040413
3. “Cannabis use was common in people with chronic non-cancer pain who had been prescribed opioids, but we found no evidence that cannabis use improved patient outcomes. People who used cannabis had greater pain and lower self-efficacy in managing pain, and there was no evidence that cannabis use reduced pain severity or interference or exerted an opioid-sparing effect.”
   Campbell, G., Hall, W. D., Peacock, A., Lintzeris, N., Bruno, R., Larance, B., … Degenhardt, L. (2018). Effect of cannabis use in people with chronic non-cancer pain prescribed opioids: findings from a 4-year prospective cohort study. The Lancet Public Health, 3(7), e341–e350. doi:10.1016/s2468-2667(18)30110-5
4. “Cannabis use disorder was associated with other substance use disorders, affective disorders, anxiety, and personality disorders.”
   Hasin, D. S., Kerridge, B. T., Saha, T. D., Huang, B., Pickering, R., Smith, S. M., … Grant, B. F. (2016). Prevalence and Correlates of DSM-5 Cannabis Use Disorder, 2012-2013: Findings from the National Epidemiologic Survey on Alcohol and Related Conditions–III. American Journal of Psychiatry, 173(6), 588–599. doi:10.1176/appi.ajp.2015.15070907
5. “These preliminary data suggest that marijuana use, especially chronic use, may affect pain response to injury by requiring greater use of opioid analgesia.”
   Salottolo, K., Peck, L., Tanner II, A., Carrick, M. M., Madayag, R., McGuire, E., & Bar-Or, D. (2018). The grass is not always greener: a multi-institutional pilot study of marijuana use and acute pain management following traumatic injury. Patient Safety in Surgery, 12(1). doi:10.1186/s13037-018-0163-3
6. “Adolescents residing in the states that had legalized medical marijuana were more likely to use cocaine/crack and heroin in the past 12 months (OR = 1.63, 2.61, respectively; both ps < 0.05).”
   Wong, S.-W., & Lin, H.-C. (2019). Medical marijuana legalization and associated illicit drug use and prescription medication misuse among adolescents in the U.S. Addictive Behaviors, 90, 48–54. doi:10.1016/j.addbeh.2018.10.017
7. “This is the largest study to date examining the association between cannabis use and illicit opioid use. Cannabis use may be a sex-specific predictor of poor response to MMT, such that women are more likely to use illicit opioids if they also use cannabis during treatment.”
   Zielinski, L., Bhatt, M., Sanger, N., Plater, C., Worster, A., Varenbut, M., … Samaan, Z. (2017). Association between cannabis use and methadone maintenance treatment outcomes: an investigation into sex differences. Biology of Sex Differences, 8(1). doi:10.1186/s13293-017-0130-1
8. “Medical cannabis laws were associated with an increase of 21.7 percent in mean age-adjusted opioid-related mortality (p < 0.0001).”
   Phillips, Elyse; Gazmararian, Julie. Implications of prescription drug monitoring and medical cannabis legislation on opioid overdose mortality. Journal of Opioid Management, [S.l.], v. 13, n. 4, p. 229-239, July 2017
9. “Not only did findings from the original analysis not hold over the longer period, but the association between state medical cannabis laws and opioid overdose mortality reversed direction from −21% to +23% and remained positive after accounting for recreational cannabis laws. We also uncovered no evidence that either broader (recreational) or more restrictive (low-tetrahydrocannabinol) cannabis laws were associated with changes in opioid overdose mortality. We find it unlikely that medical cannabis—used by about 2.5% of the US population—has exerted large conflicting effects on opioid overdose mortality. A more plausible interpretation is that this association is spurious. Moreover, if such relationships do exist, they cannot be rigorously discerned with aggregate data. Research into therapeutic potential of cannabis should continue, but the claim that enacting medical cannabis laws will reduce opioid overdose death should be met with skepticism.”
   Chelsea L. Shover, Corey S. Davis, Sanford C. Gordon, Keith Humphreys, “Association between medical cannabis laws and opioid overdose mortality has reversed over time” Proceedings of the National Academy of Sciences Jun 2019, 201903434; DOI: 10.1073/pnas.1903434116

Car Accidents

1. “There is substantial evidence of a statistical association between cannabis use and increased risk of motor vehicle crashes.”
   National Academies of Sciences, Engineering, and Medicine. 2017. The Health Effects of Cannabis and Cannabinoids: The Current State of Evidence and Recommendations for Research. Washington, DC: The National Academies Press. https://doi.org/10.17226/24625.
2. “The vast majority of epidemiological studies indicate that cannabinoids constitute the most frequently encountered psychoactive substances in the blood of drivers who are drug-impaired and/or involved in accidents, and thus cannabis must be considered a serious problem for traffic safety.”
   Bondallaz, P., Chtioui, H., Favrat, B., Fornari, E., Giroud, C., & Maeder, P. (2017). Assessment of Cannabis Acute Effects on Driving Skills: Laboratory, Simulator, and On-Road Studies. Handbook of Cannabis and Related Pathologies, 379–390. doi:10.1016/b978-0-12-800756-3.00045-4
3. “Since the legalization of cannabis, THC positivity among MVC fatalities has tripled statewide, and THC positivity among patients presenting to the highest level trauma center has doubled.”
   Steinemann, S., Galanis, D., Nguyen, T., & Biffl, W. (2018). Motor vehicle crash fatalaties and undercompensated care associated with legalization of marijuana. Journal of Trauma and Acute Care Surgery, 85(3), 566–571. doi:10.1097/ta.0000000000001983.
4. “Alcohol use and marijuana use are each associated with significantly increased risks of fatal crash involvement. When alcohol and marijuana are used together, there exists a positive synergistic effect on fatal crash risk on the additive scale.”
   Chihuri, S., Li, G., & Chen, Q. (2017). Interaction of marijuana and alcohol on fatal motor vehicle crash risk: a case–control study. Injury Epidemiology, 4(1). doi:10.1186/s40621-017-0105-z
5. “There are statistically significant life-taking effects from marijuana use by the very youngest drivers.”
   Welki, A. M. and Zlatoper, T. J. (2017). An Analysis of Illicit Drug Use and Motor Vehicle Fatalities Using Contiguous State-Level Data. Journal of the Transportation Research Forum 56(2).
6. “Alcohol and marijuana each play a significant role in fatal crash initiation. When used in combination, alcohol and marijuana appear to have a positive interaction effect on the risk of fatal crash initiation on the additive scale.”
   Li, G., Chihuri, S., & Brady, J. E. (2017). Role of alcohol and marijuana use in the initiation of fatal two-vehicle crashes. Annals of Epidemiology, 27(5), 342–347.e1. doi:10.1016/j.annepidem.2017.05.003
7. “Data analysis showed that the legalization of retail sales in these three states [Colorado, Oregon, and Washington] was associated with a 5.2% higher rate of police-reported crashes, compared to neighboring states that did not have legal retail sales of marijuana.”
   Monfort, S. S. (2018). Effect of Recreational Marijuana Sales on Police-Reported Crashes in Colorado, Oregon, and Washington. Insurance Institute for Highway Safety