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.
This study published in Monday’s American Journal on Drug and Alcohol Addiction examined the scientific literature surrounding neurodevelopment. Some public health advocates want to raise the age of legal access to cannabis to 25, despite most states prohibiting people under 21 from consuming adult-use products.
Researchers affiliated with Doctors for Drug Policy Reform have concluded, however that the proposals they made would not be effective in preventing adverse mental outcomes for consumers.
The neuroscience does not back up the idea that age 25 is a good marker for brain maturity. Cannabis policy should be based on evidence and fairness rather than mythology.
Based on the current evidence, it is socially and scientifically acceptable for an MLA to be between 18-21 years old.
This paper claims that “no endpoint can be empirically determined for neurodevelopment at 25 years of age,” because brain maturation is a “nonlinear and region-specific process” affected by the sex as well as specific physiological processes.
It states that “existing evidence shows no greater cognitive or neurophysiological damage in the long term due to cannabis consumption among individuals between 18 and 25 years of age compared with those over 25.”
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.”
It says that other, subtler changes in development continue into the third decade. The often-cited claim that brain development ‘ends’ at 25 is not clearly supported by primary neuroscientific literature,” it says.
There is no evidence that supports the widespread claims. [a minimum legal age] “Above 21 years,” the article continues. “While subtle brain development continues into the third decade of life, most key milestones are reached by age 18–21.”
The data available do not indicate that use of cannabis between the ages of 21-25 is associated with irreversible or severe harm when compared to those who begin after age 25. However, due to the rapid change in market conditions and increased consumption of cannabis among young adults as well as the high THC content, more research is required to examine the longitudinal patterns of cannabis usage, neuroanatomical, neurophysiologic, and neurocognitive effects, and to compare the harms of different ages.
While the United States has set a general age restriction of 21 years for buying cannabis, some foreign countries, such as Canada or Germany, have set it lower at 18 years.
While proponents argue that raising the legal age for underage drinking would reduce brain problems in younger people, research has consistently shown that just the policy of legalization alone deters the use of alcohol by minors.
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.” In contrast, the rates of past-year marijuana use, as well as cannabis use disorders, in younger Americans “remained constant between 2021-2024.”
Research suggests that in the United States, marijuana usage by youth has decreased across the board when the drug is legalized 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 National Institute on Drug Abuse’s (NIDA) Monitoring the Future Survey is one of the data sources cited in the report.
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. The perception of youth in 2024 that it is easy to get cannabis has also dropped significantly despite an expanding adult-use industry.
Mike Latimer is the photographer.