A decade after legalization in Colorado, one word—cannabis—now refers to a diverse array of products, some delivering a "high" from delta-9 tetrahydrocannabinol or THC at far higher concentrations than those previously used. Here, I offer a perspective on the implications of today’s cannabis marketplace for public health, and the current activities by the Colorado School of Public Health to address them. 

Breaking Down Cannabis

Cannabis refers to the dried leaves, flowers, stems, and seeds harvested from the Cannabis sativa L plant. The main compound or cannabinoid in cannabis that affects the brain is delta-9 tetrahydrocannabinol, or THC. When THC-containing products are used, THC activates CB1 receptors in the brain, which are part of the endocannabinoid system. This system is a network of receptors found throughout the body that helps regulate essential functions like mood, memory, appetite and pain sensation. That activation triggers the “high” sought by many cannabis users and reduces pain sensitivity, a therapeutic benefit. The amount of THC that reaches the brain depends on the type of product used and how it’s consumed, including the dosage and frequency of use. Regular, heavy cannabis use can alter the endocannabinoid system, leading to the development of tolerance to THC. Cannabidiol, or CBD, is another cannabinoid found in cannabis and also in hemp. It is now widely available, even in many states where cannabis has not been legalized. It does not bind to the CB1 receptor and is marketed as offering therapeutic benefits without the psychoactive effects of cannabis. This commentary will focus on products with THC, and not CBD-only products.   

Health Concerns 

Cannabis is used as a “recreational drug” and for medicinal purposes. It was first legalized for medicinal use by the State of California in 1996 under Proposition 215, the Compassionate Use Act. One of the most authoritative reviews on cannabis and health was conducted by the National Academies in 2017. It found both benefits and harms associated with cannabis use. The evidence was considered conclusive for cannabis improving chemotherapy-induced nausea and vomiting and substantial for effectiveness in treating chronic pain. The report also covered a wide range of potential adverse effects on physical and mental health, with very mixed findings on the strength of evidence for the many harms considered. The evidence was particularly strong for adverse effects on mental health, including the development of schizophrenia and other psychoses, as well as the worsening of symptoms in individuals with mental health disorders. There was also substantial evidence that cannabis use by pregnant mothers reduces newborn birth weight. As a general conclusion, the risk of adverse consequences was found to increase with the amount of THC taken into the body. Impaired driving is of concern, especially given the current high prevalence of use. Studies using driving simulators show that cannabis impairs performance. However, the impact of cannabis legalization on driving safety indicators has not yet been convincingly demonstrated in the United States highway safety data. 

Cannabis is known to cause cannabis use disorder and cannabinoid hyperemesis syndrome. The former refers to addiction to cannabis, which develops in about 10% of users. The latter, which occurs in long-term users, involves stomach pain and vomiting episodes that can last several days. The treatment for both problems: stop using cannabis.  

Legalization of Cannabis 

Cannabis and Colorado became connected in the public’s mind in November 2012 when a ballot measure—Colorado Amendment 64—was passed to amend the Constitution of the State of Colorado. It offered a new statewide drug policy for recreational cannabis. The amendment (Section 16 Personal use and regulation of marijuana) begins “(a) In the interest of the efficient use of law enforcement resources, enhancing revenue for public purposes, and individual freedom, the people of the state of Colorado find and declare that the use of marijuana should be legal for persons twenty-one years of age or older and taxed in a manner similar to alcohol.” Legal recreational sales began in Colorado and Washington State in 2014. Colorado’s pioneering status with cannabis quickly led to notoriety for the state, as captured by New York Times columnist Maureen Dowd in her memorable 2014 commentary: “Don’t Harsh Our Mellow, Dude.” 

The legalization of cannabis began a new chapter in the long saga of the plant in the United States, a story marked by shifting public attitudes and governmental approaches over the last century. Now, 38 states, four US territories, and the District of Columbia permit medical cannabis, while 24 states, three US territories, and the District of Columbia have legalized recreational cannabis. Public perception of cannabis has cycled over time, as chronicled by Emily Dufton in the appropriately titled Grass Roots: The Rise and Fall and Rise of Marijuana in America. Prior to the 1960s, when cannabis use became widespread in the United States and linked to the social movements of the time, it had been freely available for therapeutic purposes dating back to the 1800s. By the early 20th century, laws effectively ended that. In 1937, the federal Marijuana Tax Act criminalized the possession or transfer of cannabis while allowing states to enforce their own laws, and tax hemp and cannabis. 

CBD can be derived from hemp, which is legally sold under the 2018 Farm Bill. The act removes hemp containing no more than 0.3% THC by dry weight from the definition of marijuana, allowing derivatives of hemp that meet that criterion to be sold legally. This legal loophole has enabled CBD-containing products to become available in most states, although their sale operates in a complex regulatory landscape.   

Dufton’s Grass Roots reminded me of the efforts to legalize cannabis during the 1970s and 1980s, led by the lobbying organization NORML (National Organization for the Reform of Marijuana Laws) and others. A critical barrier was the Controlled Substances Act, part of the 1970 Comprehensive Drug Abuse Prevention and Control Act, which classified marijuana as a Schedule I substance alongside heroin, LSD, and peyote. This classification, which remains in effect today, made cannabis illegal at the federal level and continues to complicate research on cannabis and THC. The Reagan administration’s strong anti-drug stance, along with the high-profile anti-drug campaigns led by President Regan and his wife Nancy, doomed progress toward cannabis legalization in the 1980s. Nonetheless, cannabis use was widespread in the early 1980s, with over 15% of the population reporting use in the past year. Reported use was particularly high among youths and young adults. In 1985, among individuals aged 15-25, the prevalence of past-year use was 40% in males and 30% in females. Cannabis use fell through the early 21st century before rising once again as medical use grew and the policy environment changed. 

Today, cannabis use is common across all age groups, with some notable trends by age group. By 2022, more than 20% of people reported using cannabis in the past month, and the number of daily and near-daily cannabis users exceeded that of daily alcohol users. Recent surveys point to rising use among middle-aged and older adults. 

The Cannabis Marketplace of Today 

This evolving regulatory framework has given rise to a dynamic cannabis market characterized by a wide variety of products, consumption methods, and higher THC concentrations in cannabis flower and other products compared to those available in past decades (Table 1). From the 1960s through the 1980s, the typical THC concentration in cannabis flower ranged from 2% to 4%. By 2022, that percentage was over 16% nationally (Figure 1). Today’s market includes a diversity of products beyond flower: edibles in various forms; vape pens delivering a THC-containing aerosol; and concentrates that are vaporized and inhaled (Figure 2). Edibles, including brownies, cookies, candies and drinks that are sold in dispensaries, can include no more than 10mg THC per serving. THC-containing inhalational devices, otherwise known as vapes, can deliver THC at concentrations as high as 70-90%. Concentrates like wax, shatter and budder, are made by processing flower to extract THC, yielding a solid with a high THC concentration. Users heat the concentrates and inhale the resulting vapor.  The profile of THC in the body differs by product, rising and peaking quickly for inhalational products (concentrates, vape pens, smoked cannabis) depending on the puffing pattern, and peaking slowly and unpredictably for edibles. 

Table 1: Cannabis products available on the marketplace and their typical THC amounts by product. 

Figure 1  

Figure 2: Cannabis vaping products used in various vaping devices. A = metered dose inhalers; B = desktop vaporiser; C = portable vaporiser; D = thermal extraction device; E = first generation disposable cannabis e-cigarettes; F = second generation cannabis e-cigarettes with prefilled or refillable cartridges; G = third generation tanks or mods; H = fourth generation pod mods; I = dab pen; J = dab rig. 

1 = metered dose cartridge; 2 = dried cannabis flower; 3 = 510-thread cartridge with e-liquid; 4 = e-liquid refill bottle; 5 = pod cartridge with e-liquid; 6 = wax; 7 = shatter; 8 = budder. *Some devices in categories B, C and D can also take E-liquid and solid cannabis concentrate products. 

Source: MacCallum, C. A., Lo, L. A., Pistawka, C. A., Christiansen, A., & Boivin, M. (2024). Cannabis vaporisation: Understanding products, devices and risks. Drug and Alcohol Review, 43(3), 732-745. 

The rising access to cannabis products with far higher THC concentrations than those previously available has raised concerns about associated risks, particularly for adolescents and young adults. These age groups are particularly susceptible to poor outcomes because brain development is not complete until age 25. There is also concern about exposure of the fetus to THC during gestation. Accidental ingestion of dangerous amounts of edibles by infants and young children is well-documented through emergency room records and poison control center reports, and such cases are on the rise. States have struggled with protective policies around these new products with much higher THC concentrations than previously, as available scientific evidence does not pinpoint a THC concentration that is too risky to be acceptable. States have adopted a number of policies, with Vermont specifying a maximum THC concentration of 60%. 

Colorado House Bill 21-1317 

Concerned by the potential health risks of the high-concentration products in the state’s cannabis marketplace, the Colorado General Assembly passed House Bill 21-1317 (HB 1317) in 2021. Among its provisions, HB 1317 called on the Colorado School of Public Health to “…conduct a systematic review of all available scientific evidence-based research regarding the possible physical and mental health effects of high-potency THC marijuana and marijuana concentrates regardless of the location of the research.” It also called for the school to carry out a public educational campaign about these cannabis products. The campaign activities were launched in 2023-2024.  

To determine what is known about these products, the school assembled a review team under my guidance as Dean. The team completed a scoping review, a comprehensive survey of the relevant published literature, to identify studies suitable for more in-depth and focused analyses. The initial review screened more than 60,000 scientific papers. Ultimately, 452 studies were selected (updated a year later to 550) that met the criteria for relevance to the critical policy question: What are the public health consequences of the availability of these newer products with higher concentrations of THC than were previously available? Overall, we found the evidence foundation profoundly lacking for addressing this critical question and supporting informed decision-making. The marketplace has moved much faster than research, and little has been published that directly relates to the array of products now available (Figure 2). We and others have called for more timely studies that use enhanced research approaches.  

The Tea on THC 

In partnership with Initium Health, a public health media firm, the Colorado School of Public Health has launched THE TEA ON THC, a public education campaign. For those generationally challenged by the term “THE TEA,” as I was, it refers to sharing inside information (and not the plant and the beverage). This new initiative educates Coloradans on the often-overlooked health risks of high-concentration cannabis use by fostering open dialogue on the subject. It provides critical resources for priority populations including young people, peers, teachers and loved ones, as well as pregnant and parenting adults.  

The campaign’s unique brand and home is The Tea on THC website and social media channels. On the website, visitors will find audience-specific web-based curricula, myths and facts, podcasts, videos, links to social media posts and an AI-powered chatbot. The videos showcase first-hand testimonials, including stories from students at 5280 High School, a recovery-focused school; a mother-turned-advocate who lost her son to suicide from cannabis-induced psychosis; and former Denver Bronco Brandon Lloyd, who experienced paranoia, mood swings, and dependency from his use of high concentration cannabis until he discovered alternative coping strategies beyond substance use. The Tea on THC online resources also include state and national resource links and strategies for reducing stress without substances. 

The campaign reaches audiences through multiple channels, programs and in-person events. It is being rolled out now. 

The Future 

Cannabis is here to stay. I anticipate that more states will allow medical cannabis and legalize recreational uses and that cannabis will be moved to Schedule III in the Controlled Substance Act. Rescheduling will not lead to mass public availability of cannabis. However, a move to Schedule III would legalize medical cannabis nationally and facilitate government-funded research, which is severely restricted by the Schedule I classification. Rescheduling would not affect state-level cannabis legalization for recreational use unless another federal decision is made addressing it. A hearing is scheduled for this month. I also anticipate that products in the marketplace will continue to diversify. Vaping THC poses a particular threat to youths because it can be done surreptitiously, similar to nicotine vaping, with high concentrations of THC. I am concerned by an emerging overlap between the use of these two drugs through vaping. We don’t know if rising use among adults will continue. 

The Colorado School of Public Health evidence review demonstrates that much more and more timely research is needed. The pace of change in products sold greatly outstrips the pace of research. The same timeliness challenge limits public health surveillance, such as tracking what products people are using and how they may be affecting them. Ironically, research and public health tracking are limited by funding, even as a multi-billion dollar cannabis industry grows. Given the dynamic marketplace and potential long-term consequences of cannabis use, potentially across most of the life course, we need careful and ongoing monitoring for adverse and unanticipated consequences of cannabis use. As a reminder, when Camel cigarettes became the first mass-marketed brand in 1913, health risks were not a consideration. The still-devastating epidemic of lung cancer was detected 30 years later. Amid major policy changes around cannabis and the rise of high-concentration products, we need to be vigilant and ensure that an avoidable public health crisis does not occur.

This article is part of a monthly column, The Jon Samet Report, on the biggest issues facing us today in public health, written by the former dean of the Colorado School of Public Health, Jon Samet, a pulmonary physician and epidemiologist, and Professor of Epidemiology and Occupational and Environmental Health. Dr. Samet is a global health leader, shaping the science and conversation on issues ranging from tobacco control to air pollution to chronic disease prevention and more. Each month he shares expert insights on public health issues from local to global.