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Today, a research team assembled by the Colorado School of Public Health (ColoradoSPH) in response to the 2021 Colorado House Bill HB21-1317, “Regulating Marijuana Concentrates,” delivered its mandated review to Colorado legislators on the scientific evidence related to the physical and mental health effects of high-concentration THC (delta-9-tetrahydrocannabinol, the main psychoactive compound in cannabis products). This review was requested as the marketplace shifted towards high-concentration products. The health implications of this change are not well understood. In a comprehensive scoping review, the team screened approximately 66,000 studies and ultimately identified 452 published through late 2022 that are relevant to understanding the health effects of high-concentration cannabis products. The ColoradoSPH team also created a first-of-its-kind interactive and publicly available evidence map of the 452 cannabis studies, which makes the studies searchable and accessible.   

“With funding from the State of Colorado, a valuable, public resource has been developed for public health and scientific purposes. The scoping review and evidence map are unique; it is the first and most complete systematic assessment of the entire body of literature related to high-concentration cannabis,” said Jonathan Samet, MD, MS, dean of the ColoradoSPH. “Since legalization, high-concentration cannabis has become a large part of the marketplace and there are concerns and unknowns about the public health implications. This scoping review and resulting evidence map are the most authoritative effort to compile the available peer-reviewed research on this to-date.”

Overall, the ColoradoSPH team found that research focusing on high-concentration cannabis products was limited. The challenges to applying the literature include lack of standardized methods for assessing use of the wide range of cannabis products, and the many different possible health effects of using high-concentration products that have been studied. Additionally, the team noted that the THC concentrations used for many past research studies were much lower than the concentrations that are typically available for purchase today. One reason for this difference is that government-funded research studies have been generally limited to lower concentration products.  

However, there was one topic area for which the researchers found a moderate amount of evidence: the effects on mental and behavioral health outcomes. Based on the review and evidence map that was created, they found some evidence that high-concentration cannabis products can affect mental health. Greg Tung, PhD, associate professor of Health Systems, Management and Policy in the ColoradoSPH commented “There was a moderate amount of evidence that high-concentration THC can have adverse effects on those with pre-existing conditions such as psychosis, but there are also studies that show beneficial outcomes from the use of high-concentration cannabis on other mental health conditions such as PTSD, anxiety, and depression.”  

Based on these and other findings, the ColoradoSPH team provided a list of recommendations for future research and next steps they plan to take. They will use the review to comment on the state of scientific literature and the need for more informative research by addressing some of the research gaps identified, such as standardizing approaches for assessing use of cannabis products. Following input from the Scientific Review Council, the team also plans to complete systematic reviews related to specific mental health outcomes. The Council also provided recommendations based on the scoping review.

“Some of the problems of the scientific literature on cannabis have been recognized, and they need to be addressed so that future research is more informative,” the team said in their executive summary. “In particular, standardized approaches are needed for characterizing the use of cannabis products to assure comparability among studies. These approaches need to be modified in a timely way so that the instruments used for research reflect current patterns of use. Systematic reviews and meta-analyses would be facilitated by such standardization. Attention to use of common methods for outcome assessment in studies of cannabis would be similarly valuable.”

The team also recommends sustained support to continually update this resource, given the rapid growth of the scientific literature, the growing availability of recreational and medical cannabis, and the rise of high-concentration products.

Per its mandate from HB 21-1317, the team did not review the whole range of public health issues related to cannabis and THC. Rather, the focus was “…on physical and mental health effects of high-potency THC marijuana and concentrates.”