The Cannabis Comeback

The number of marijuana users has more than doubled in the past decade as legalization has spread across the United States.

November 10, 2020

The last session on the last day of any scientific conference is always quiet, and professor of epidemiology (in Psychiatry) Deborah Hasin, PhD, was anticipating a sparse crowd when she took the podium at an American Psychiatric Association meeting in May, 2010. To her astonishment, the room was packed, and at the conclusion of her presentation on marijuana dependence, the attendees asked a seemingly endless stream of questions. “We could barely get people out of the room,” Hasin recalls.

Many of the questions were about the new laws legalizing marijuana use that were sweeping the nation, and what their effect might be on usage of the drug. By 2010, 13 states had enacted laws that allowed citizens to use marijuana—or more specifically the leaf of the cannabis plant, or extracts thereof—for medical purposes. A few had passed further legislation to allow recreational marijuana use as well.

Since then, medical marijuana has been legalized in 33 states and Washington, D.C.; recreational marijuana is now legal in 11. At press time, a bill to decriminalize marijuana at the federal level was gaining momentum in Congress. And more Columbia Mailman School researchers, including Pia M. Mauro, PhD, assistant professor of epidemiology, and Silvia Martins, MD, PhD, associate professor of epidemiology, have published extensively on the public health impacts of marijuana use, and become among the foremost experts on the subject.

Today’s marijuana is not the pot that previous generations smoked: The products widely available now are tremendously more potent. Many have a concentration of THC (tetrahydrocannabinol, the active ingredient in cannabis plants) of 20 percent or more, compared to 3 percent two decades ago. And while smoking remains a popular way of ingesting the drug, other, higher-potency modes of consumption such as edibles, vaping, and dabbing—inhaling vapor—are gaining ground. Although the substance remains illegal under federal law, public opinion has largely shifted to support legalization, and with the $17 billion (and swiftly growing) cannabis industry pushing for it, the trend is almost certain to continue. “At this point, the question is not whether remaining states are going to be enacting these laws, but when,” says Mauro.

Epidemiologists are only beginning to understand the effect of state legalization laws and other cannabis-related policy measures on public health—not just on marijuana use, but also on downstream consequences, such as how people who use cannabis interact with the criminal legal system, or whether people who use cannabis are also more likely to use other drugs, including alcohol. “Unfortunately, policies are too often enacted with little or no consideration of the public health effects,” says Mauro. As she and her colleagues have contributed to building that knowledge base, presenting their work through the media or directly to policymakers, states can rely on it to craft policies that mitigate the risks that marijuana can pose—and to reshape legalization laws as well.

When Hasin started looking at the public health effects of state marijuana policies, she quickly noticed that rates of cannabis use were higher in states that had adopted medical marijuana laws. Researchers now know that the effect is causative, not coincidence. A major concern at the time was whether these laws would increase usage in teens, who are especially at risk of mental health issues as well as cognitive and motivational problems from the drug. Contrary to their expectations, Hasin and her colleagues found that in states where marijuana was legal in some form, teens across different age groups didn’t use the substance more frequently—and in fact eighth graders seemed to use it less. Subsequent studies, by Hasin and others, have supported this conclusion. “Most of the data haven’t shown a huge increase in adolescent use after the passage of legislation allowing both medical and recreational use,” says Martins, who is also director of the Substance Abuse Epidemiology Unit.

In general, studies show that legalizing medical marijuana does not seem to boost the prevalence of recreational use of the drug. However, what’s becoming increasingly clear is that states that pass recreational cannabis laws do see a significant uptick in cannabis use disorder among adults—as well as a small bump in cannabis use disorder among teens who were already using the drug. That finding came through loud and clear in a study of about 500,000 people published in JAMA Psychiatry last year by Martins and colleagues. Frequent use (defined in the study as more than 20 days out of a month) increased too, as did the number of people who were classified as having cannabis use disorder (the use of marijuana despite clinically significant impairment of psychological, physical, or social functioning). The COVID-19 pandemic has added an extra layer of urgency to cannabis research: The latest Global Drug Survey, an annual study of drug-use trends, which polls some 55,000 people in 11 countries, found that almost two in five people had increased their use of cannabis since the start of the pandemic. Many cited stress and depression as reasons underlying their increased use.

Martins’ findings suggest that states that allow recreational use must also expand access to drug treatment. Arrests for marijuana possession have long been a manifestation of racial bias in policing, and one major argument in favor of legalization is that it decreases contacts with the criminal justice system in communities of color. On top of this social justice argument, proponents say that the marijuana market can also generate jobs and tax revenue for local communities. “But there are always tensions between these benefits and public health issues,” Hasin says.

In research that Mauro and colleagues published online in the Journal of Studies on Alcohol and Drugs, “We found significant increases in daily cannabis use across adult age categories after 2007,” she says. “This could be due to increasingly permissive cannabis legislation and lower risk perception.” This and other data are worrying. National data show that the number of people in the U.S. who use marijuana daily has doubled to about 8 million between 2008 and 2018, according to Guohua Li, DrPH, MD, Finster Professor of Epidemiology and Anesthesiology and the founding director of the Center for Injury Epidemiology and Prevention. “That’s a pretty troubling sign because daily use is a strong indicator of dependence,” says Li.

Over the past few years, Li’s team has demonstrated that marijuana use, especially in combination with alcohol, is associated with a marked increase in the risk of fatal car crashes. Meanwhile, studies consistently show that daily marijuana users have a heightened risk of mental health issues such as psychosis, and recent work Li has done with his colleagues suggests that frequent use is tied to an increased risk of being charged with a felony.

Designing and implementing legalization thoughtfully could help stem its negative effects. Columbia Mailman School researchers are now working to further understand how recreational or medical legalization laws, which vary widely state to state, influence health outcomes. For example, what is the impact when states have robust prevention programs—particularly for teens—as well as treatment programs? How do the potency and types of marijuana products the state allows affect health outcomes? As cannabis becomes big business, it is critical to look for “ways to legalize marijuana and still protect the most vulnerable,” says Martins.

Hasin, meanwhile, is exploring how marijuana use interfaces with the risk of COVID-19—for example, whether users have a higher risk of infection, and whether stay-at-home orders have contributed to an increase in use. She is also trying to pin down whether people who suffer from chronic pain or psychiatric conditions such as depression or post-traumatic stress disorder might be more at risk of increasing their marijuana use to unhealthy levels in response to changes in state laws. Earlier this year, Renee Goodwin, PhD, MPH ’03, adjunct associate professor in the Department of Epidemiology, published research showing that pregnant women with depression are more than three times more likely to use cannabis than those without depression. Pregnant teens were especially vulnerable. “Our findings are timely, given rapidly shifting perceptions about risks associated with cannabis use and its legalization,” Goodwin noted.

Mauro is looking at whether state legalization laws may change how people who frequently use the drug interact with the criminal legal system and how easily they are able to access treatment. For example, she explains, if someone is less likely to be arrested for possessing marijuana, they may also be less likely to be referred to treatment as a consequence of the arrest. “It’s not entirely clear whether these treatment episodes were unnecessary to begin with, or whether we are reducing the ways people get into treatment. The question is, how do we create a system that doesn’t rely on the criminal legal system to access health services?” Mauro says. These things are hard to parse, she acknowledges, but that is one of the powerful aspects of this work. “We try to isolate effects that are very much intertwined—and what I love about public health is that we acknowledge that complexity.”

Curious to learn more about the impact between cannabis and public health issues? Check out this Columbia Mailman research detailing the relationship between cannabis use and non-medical opioid use in the US.

Alla Katsnelson’s work has appeared in Chemical & Engineering News, BBC Science Focus Magazine, The New York Times and other publications. She has a PhD in mammalian brain development.