The theory that marijuana use can negatively—and potentially permanently—rewire the brain up until a person reaches the age of 25 is based on misleading science that neglects to account for key factors in cognitive maturity, according to a new research paper.
The study, published on Monday in the American Journal on Drug and Alcohol Abuse, examined the scientific literature around neurodevelopment. While most U.S. states prevent people under 21 to access adult-use cannabis products, some public health advocates are pushing to the legal age limit to 25.
But the researchers, who are affiliated with the advocacy group Doctors for Drug Policy Reform, concluded that those proposals would not meaningfully prevent adverse mental health outcomes for consumers.
“Invoking age 25 as a bright line for brain maturity is not supported by neuroscience,” they wrote. “Cannabis policy should reflect evidence and fairness, not mythology.”
“Based on current evidence, an MLA between 18 and 21 is both scientifically sound and socially defensible.”
The paper states that there’s “no empirically defined neurodevelopmental endpoint at age 25,” as brain maturation “is a nonlinear process, region-specific, influenced by sex and specific physiological processes.”
“Importantly, existing evidence does not demonstrate greater long-term cognitive or neurophysiological harm attributable to cannabis use in individuals aged 18-25 years compared to those older than 25,” it says.
The researchers reviewed data on the macrostructural and microstructural development of the brain, which shows that such maturation is “mostly complete by the end of adolescence, around age 18.”
“Other, more subtle developmental changes continue throughout the third decade of life. The often-cited claim that brain development ‘ends’ at 25 is not clearly supported by primary neuroscientific literature,” it says.
“Despite widespread assertions, there is no clear neurobiological or patient-centered evidence to support [a minimum legal age] above 21 years,” it continues. “While subtle brain development continues into the third decade of life, most key milestones are reached by age 18–21.”
“Available data do not show that cannabis use between ages 21 and 25 confers uniquely severe or irreversible harm compared to use beginning after 25. Nevertheless, given the rapidly changing market conditions, the increasing use of cannabis by young adults, and the higher THC concentrations being consumed, research is needed to further examine longitudinal trajectories of cannabis use, neuroanatomic, neurophysiologic and neurocognitive outcomes, and comparative harms across ages to refine evidence-based recommendations for MLAs that minimize risks while avoiding unintended social consequences.”
For what it’s worth, although U.S. states have broadly imposed a 21-year-old age limit to buy cannabis for adult use, other foreign jurisdictions such as Canada and Germany have set the limit younger, at 18.
And while proponents of raising the age limit argue that it would mitigate brain issues with younger people, studies have also consistently indicated that the very policy of legalization itself has deterred underage use.
For example, a recent federally funded study out of Canada shows that youth marijuana use rates have declined after the country legalized cannabis—contradicting concerns voiced by prohibitionists.
The study was released about three months after German officials released a separate report on their country’s experience with legalizing marijuana nationwide.
That report found that fears from opponents about youth use—as well as traffic safety and other concerns—have so far proved largely unfounded.
A separate recent study conducted by German federal health officials also found that rates of marijuana use declined among youth after the country legalized adult-use cannabis, contradicting one of the more common prohibitionist arguments against the reform.
Back in July, federal health data also indicated that while past-year marijuana use in the U.S. overall has climbed in recent years, the rise has been “driven by increases…among adults 26 years or older.” As for younger Americans, rates of both past-year use and cannabis use disorder, by contrast, “remained stable among adolescents and young adults between 2021 and 2024.”
Across the U.S., research suggests that marijuana use by young people has generally fallen in states that legalize the drug for adults.
A report from the advocacy group Marijuana Policy Project (MPP), for example, found that youth marijuana use declined in 19 out of 21 states that legalized adult-use marijuana—with teen cannabis consumption down an average of 35 percent in the earliest states to legalize.
The report cited data from a series of national and state-level youth surveys, including the annual Monitoring the Future (MTF) Survey, which is supported by the National Institute on Drug Abuse (NIDA).
The latest version of the MTF, released late last year, found that cannabis use among eighth, 10th and 12 graders is now lower than before the first states started enacting adult-use legalization laws in 2012. There was also a significant drop in perceptions by youth that cannabis is easy to access in 2024 despite the widening adult-use marketplace.
Photo courtesy of Mike Latimer.
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