The Trump administration's recent move to reclassify some cannabis products as less dangerous has been hailed by advocates who argue cannabis shouldn't be treated the same as other, more deadly drugs such as heroin or ecstasy. But research points to a growing concern around how cannabis affects the developing teenage brain in ways that may have dangerous, long-term ramifications.

On April 23 the administration downgraded state-licensed medical marijuana products from Schedule I to Schedule III. (Drugs in Schedule I have no medical use and high potential for abuse, and those in Schedule III have moderate to low potential for physical and psychological dependence, according to the U.S. Drug Enforcement Administration.) More regulation to lower barriers to researching cannabis for potential medicinal use could come soon—including further investigating how marijuana products might affect teens.

Here’s what scientists know so far about the effects of cannabis on the teenage brain.

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Cannabis Accelerates Thinning of the Cerebral Cortex

Accumulating research on cannabis use among teenagers suggests it may alter their brain’s structure in measurable ways. Findings reported in June 2021 in JAMA Psychiatry suggest that when teens use cannabis, it causes increased thinning of the cerebral cortex, the outermost layer of the brain, compared with those who don’t use it or use it less. The cortex is composed of gray matter that’s responsible for high-level thinking, which plays a role in learning, memory, problem-solving, emotions and consciousness. The study followed nearly 800 adolescents who had their brain imaged prior to using cannabis at an average age of 14, and then five years later after they reported having used the drug in varying amounts.

As we age, our brain undergoes natural cortical thinning, which results in a reduction in gray matter, says lead study author Matthew D. Albaugh, an assistant professor at the University of Vermont in Burlington. The brain changes they saw in young people who used cannabis resembled those normally associated with aging. “The more [cannabis] use that these youths were reporting, the faster thinning was occurring in certain prefrontal areas,” Albaugh says. This may be driven by synaptic pruning, or the brain’s elimination of unused neural connections, which is thought to accelerate cortical thinning, he says.

The research is observational, so it’s difficult to know the amount of thinning that might be the result of preexisting health conditions or other factors. Still, the study included a large number of teens. And researchers have seen similar cannabis-driven brain changes in studies in which they exposed animals to the drug, Albaugh says.

Cannabis Can Be Habit-Forming to the Teenage Brain

Cannabis use affects connectivity in several different brain areas, including the cortical regions, the hippocampus (a region involved in memory formation) and limbic striatal networks (circuits involved in emotion and reward processing), according to a study published in the journal Neuropsychopharmacology.

“These parts of the brain are important because we think that they’re also involved in things like addiction and psychosis, both of which are potentially issues with cannabis use,” says the study’s senior author Matthew Wall, a psychologist and neuroscientist at Imperial College London.

Another factor driving cannabis use disorder among teens, experts say, is that cannabis has become much more potent than it once was. Cannabis use disorder wasn’t nearly as common 20 years ago, but today it affects an estimated 30 percent of people who use cannabis in the U.S. Wall says that growing these plants hydroponically with intensive selection for stronger and stronger strains has dramatically increased the risk of becoming dependent on the drug. Some cannabis products are up to 95 percent THC.

Two decades ago most marijuana was about 3 or 4 percent THC. “It’s so much stronger now that it’s almost a different drug,” Wall says. The increased potency can cause withdrawal symptoms, including irritability, depression, restlessness, headache, loss of appetite, insomnia and severe drug cravings.

Cannabis Can Cause a Severe Condition Called Psychosis

Cannabinoid receptors are also prevalent in the hippocampus, which may be part of the reason research has shown a high association between cannabis use in the teenage years and poor academic performance, impulsivity and self-regulation, adverse emotional states and low social engagement, according to a study published last year in Pediatrics.

Ryan Sultán, an assistant professor of clinical psychiatry at Columbia University and lead author of the study, contends that changes in the hippocampus can also result in the most serious condition, psychosis, characterized by delusions, hallucinations and paranoia, is likely a downstream effect of the release of high levels of the pleasure-signaling chemical dopamine resulting from dysfunction in the cannabinoid receptor system. A study published earlier this year in JAMA Health Forum found that adolescent cannabis use was associated with increased risk of psychotic and bipolar disorders.

“If you’re a cannabis user [as a teenager] and we follow you over time, you’re much more likely to develop a psychotic disorder, depression or anxiety,” says Sultán, who was not involved in the JAMA Health Forum study.

Cannabis Reduces Signaling in the Endocannabinoid System

When researchers look at how cannabis affects the brain, they see the most impact during adolescence, when the brain is already undergoing so many other changes.

The endocannabinoid system—a brain system that regulates mood, sleep, memory and appetite—also undergoes substantial growth and signaling activity during adolescence.

When you bring in an outside cannabinoid product such as cannabis, the endocannabinoid system tamps down its own signaling, says that study’s lead author Natasha Wade, principal investigator at the Neuropsychology, Development, and Toxicology Lab at the University of California, San Diego.

“Why would the brain make its own signals when it’s provided them externally?” Wade says.

THC in particular binds to cannabinoid receptor 1 (CB1). And less signaling in CB1 may impair cognition and memory and induce anxiety and depression, studies suggest.

And if a teenager is already prone to depression and anxiety, and marijuana becomes a temporary solution to the problem, it can cause dependency at a time when that young person should be developing healthy coping mechanisms. Teens end up creating a cycle in which they’re constantly “chasing their tail,” Sultán says—where using marijuana to treat mental health issues only exacerbates the problems they’re trying to address.