Alcohol and marijuana are among the most commonly used drugs by adolescents and young adults. The question of whether these two drugs are substitutes or complements has important implications for public policy and prevention strategies, especially as laws regarding the use of marijuana are rapidly changing. Researchers examined the deaths of nearly 7,200 drivers, aged 16-25, in crashes that occurred between 1999 and 2011 in California, Connecticut, Hawaii, Illinois, New Hampshire, New Jersey, Rhode Island, Washington state and West Virginia. These states routinely conduct blood and urine tests of drivers who die in crashes.
Overall, 50.5% of the drivers studied tested positive for alcohol or marijuana. Univariable relative risk modeling revealed that reaching the minimum legal drinking age was associated with a 14% increased risk of alcohol use (RR = 1.14, 95% CI: 1.02 to 1.28), a 24% decreased risk of marijuana use (RR = 0.76, 95% CI: 0.53 to 1.10), and a 22% increased risk of alcohol plus marijuana use (RR=1.22, 95% CI: 0.90 to 1.66). Study leader Katherine Keyes, an assistant professor of epidemiology at Columbia University’s Mailman School of Public Health, said “Policies related to the use of substances in the United States remain in flux; the rapid changes in marijuana use policy are a good example of this... It’s imperative to know whether there will be unintended consequences of changes in policies, including increases or decreases in harm related to other substances that are not the focus of the policy,” she added.
Study co-author Dr. Guohua Li, director of Columbia’s Center for Injury Epidemiology and Prevention, said “Taken together, we found no significant substitution effect between alcohol and marijuana. Rather, an uptick in availability seems to increase the prevalence of concurrent use of alcohol and marijuana.”
Source: Effects of minimum legal drinking age on alcohol and marijuana use: evidence from toxicological testing data for fatally injured drivers aged 16 to 25 years Katherine M Keyes, Joanne E Brady and Guohua Li. Injury Epidemiology 2015, 2:1 doi:10.1186/s40621-014-0032-1.