When you light up or down an edible, you may be lowering your risk of cognitive decline, according to a new study comparing recreational cannabis users to nonusers. As marijuana isn’t without its health harms, these findings came as a surprise even to the scientists behind the study.
Researchers at the State University of New York Upstate Medical University found that nonmedical cannabis use—regardless of how or how often it was consumed—lowered a person’s odds of subjective cognitive decline (SCD) by 96%. The results were published in February in the journal Current Alzheimer Research.
“I was expecting cannabis to be linked to an increased risk for cognitive decline, because that’s pretty much what’s consistent in previous research,” study coauthor Roger Wong, Ph.D., an assistant professor of public health and preventive medicine at the university’s Norton College of Medicine, tells Fortune. “I was stunned by the opposite finding.”
Dual use of cannabis, for both medical and nonmedical purposes, as well as medical use alone also correlated to decreased risk of SCD, the self-reported worsening or increased frequency of confusion or memory loss. However, those associations weren’t statistically significant.
Previous research suggests people with SCD are 2.5 times more likely to develop dementia and 1.8 times more likely to develop mild cognitive impairment. About one in nine U.S. adults ages 45 and older experience SCD, according to the Centers for Disease Control and Prevention (CDC).
“We don’t have a way to prevent dementia right now,” Wong says. “But if we can prevent subjective cognitive decline at the very beginning and track it, that’ll hopefully fix some of the issues that we’re having right now with dementia later in life.”
SCD most common among medical cannabis users
Wong and public health graduate student Zhi Chen used the CDC’s 2021 Behavioral Risk Factor Surveillance System (BRFSS) to study nearly 4,800 U.S. adults 45 and older. To indicate SCD, the comprehensive health survey included the question, “During the past 12 months, have you experienced confusion or memory loss that is happening more often or is getting worse?”
In terms of respondents’ cannabis use, Wong and Chen examined:
- Frequency of use in past month: 0–30 days
- Reason for use: nonuser, medical, nonmedical, both medical and nonmedical (dual)
- Method of consumption: nonuser, smoke, eat, drink, vaporize, dab, or other
Researchers weighted the sample size so that the nearly 4,800 respondents represented more than 563,000 people. About 53% were women, 46% were Asian, and 16% were 60–64 years old, a larger proportion than any other age group. A third of respondents ranked their health “very good.” Of the 8% who used cannabis, a 3% plurality did so for medical purposes. Smoking was by far the most common consumption method and, on average, respondents had used cannabis 1.4 of the previous 30 days.
Among cannabis users with SCD, cognitive decline was most common in people who used cannabis for medical reasons, followed by dual and recreational users. Further data analysis showed a statistically significant association between nonmedical use and 96% decreased odds of SCD.
Cannabis-sleep-dementia connection
Two caveats may explain Wong’s results, which show correlation, not causation, he tells Fortune. One is that much of the existing literature on cannabis and cognition focuses on frequency of use; Wong did find that more frequent cannabis use correlated to cognitive decline, but the association wasn’t statistically significant. Second, previous research has shown cannabis use to be detrimental to adolescents, whereas this study focuses on middle-aged and older adults, he says.
“I tried to expand on previous research by not just looking at frequency, but you also need to consider why they’re using cannabis and how they’re using cannabis,” Wong says, “because there’s different chemicals, different compounds in nonmedical and medical cannabis that I think is really crucial.”
Dr. Brooke Worster, an assistant professor in the M.S. in Medical Cannabis Science and Business program at Thomas Jefferson University, clarifies that cannabis composition may be a more impactful variable on cognitive decline than whether it’s intended for medical or recreational use.
“The thing that would be awesome to know is, ‘What concentration or what percentage of THC versus CBD was in what you’re using?’” Worster tells Fortune. “That’s the problem in general, is that the vast majority of adults that use cannabis for any reason don’t really know what the composition of it is.”
The very catalyst for Wong’s research may also play an important role: sleep. In a study published last year in the American Journal of Preventive Medicine, he had found that sleep difficulties may raise a person’s risk for dementia. Wong says he received feedback from several people who said they used cannabis to help them fall asleep and wanted to know if doing so was increasing their risk for dementia.
“Let’s say, if someone is using it—whether they perceive it as medical or not—it helps them relax and enjoy a Friday night. It also helps them relax and to go to sleep,” Worster says. “You could make the argument that in some ways, that’s health- or medical-related.”
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US cannabis laws pose research obstacle
A major limitation of Wong’s study is that the legalities of cannabis use vary greatly from state to state. And because the data span 2021 alone, more longitudinal research is necessary, Wong says. In addition, he suspects BRFSS may underestimate cannabis use.
“We’re relying on people to self-report whether or not they’re using cannabis,” Wong tells Fortune. “There might be some sort of bias depending on whether they’re living in a state right now where cannabis is illegal for either medical or nonmedical reasons.”
Researchers also are relying on survey respondents’ interpretation of their own cognitive health, Worster stresses, referring to the subjectivity inherent in the definition of SCD. Future studies involving objective cognitive decline may be beneficial.
“People either think they’re doing great or are really concerned,” Worster tells Fortune. “You’d want to get a cohort of people to do the same reporting and give them a test that [shows] an objective sense of, are they thinking that their memory is bad and it’s not showing that, or vice versa.”
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