Cannabis, or marijuana, is commonly used by American teenagers. In 2022, according to the Centers for Disease Control and Prevention (CDC), 30.7% of US 12th graders had used cannabis in the past year, and 6.3% had used it daily in the past 30 days.
Whether that matters more at 16 than at 30 is a reasonable question. The teen brain is still being built, and it keeps developing until around age 25. Tetrahydrocannabinol (THC), the ingredient in cannabis that causes the high, acts on a system in the brain that helps direct that building work during the teen years.
That does not mean every teenager who uses cannabis will be harmed. It does mean the timing is worth taking seriously.
The Brain Keeps Developing Until About Age 25
The CDC notes that the teen brain is actively developing and continues to develop until around age 25. Not every part finishes at the same time. One of the last regions to mature is the prefrontal cortex, the area behind decision-making, impulse control, attention, planning, and working toward goals, as described in a 2020 PubMed Central article on cannabis and the adolescent brain.
Two things happen to the brain during these years.
- Pruning: A young brain has more connections between cells than it needs. Through adolescence it clears out the ones that go unused and strengthens the ones that get used, a bit like trimming a hedge so what remains grows better. This is thought to make communication between brain cells more efficient and help the brain move from an immature state to an adult one.
- Myelination: The long fibers that connect distant parts of the brain get wrapped in a fatty coating called myelin, which works like the insulation on a wire. Signals travel faster, and separate regions coordinate with each other better.
Both of these are guided by something called the endocannabinoid system, a signaling network the body produces on its own. StatPearls describes it as guiding pruning and myelination during adolescence. It got its name because the chemicals the body makes for it are chemically similar to compounds in cannabis.
That is the overlap that makes the timing question worth asking. Brain cells carry receptors, which are docking points that specific chemicals fit into, and that is how one cell passes a signal to another. THC fits into a receptor called CB1, the same one the body’s own version of these chemicals is built for. The drug acts on machinery that, during the teen years, is still in the middle of a job.
Why THC Affects a Teenage Brain Differently
Most of what follows comes from animal research. Researchers cannot ethically give THC to teenagers to see what happens, so the detailed biology has been worked out in rats and mice. That is a real limit on how far the findings can be pushed. What the animal work suggests is that a developing brain does not respond to THC the way a finished one does.
- The system THC acts on is still changing. Blest-Hopley and colleagues, reviewing the evidence, note that CB1 receptors in the outer layers of the brain steadily increase through adolescence before settling at adult levels, while the deeper reward and habit regions follow the opposite pattern. THC arrives in the middle of that shift rather than after it.
- The teenage brain adjusts to it less. Normally the brain compensates for cannabis by reducing its CB1 receptors, which is a way of turning down the volume on repeated stimulation. In the animal studies Blest-Hopley and colleagues reviewed, adolescent animals showed less of this compensating response than adults did after the same exposure.
- There appears to be a window. Rat research described in a 2020 PubMed Central article found that a THC-like drug given during early and mid adolescence, but not during late adolescence or adulthood, interfered with GABA, a chemical that calms brain activity, in the prefrontal cortex. Those animals reached adulthood without developing the electrical activity patterns typical of a mature brain. The researcher behind that work estimates the equivalent human cutoff at around 16 or 17.
- The reward system takes a heavier hit. THC produces its pleasurable effect by driving up dopamine, the brain’s reward chemical. Fischer and colleagues describe how chronic use reverses that, reducing dopamine activity and the density of dopamine cells in a small region deep in the brain that supplies most of the brain’s dopamine. They note these reductions are more pronounced with adolescent use than with adult use, which they link to blunted enjoyment, low motivation, and depressed mood.
What Human Studies Show: Cognition and Brain Structure
Animal research can separate cause from effect. Human research on teenagers cannot, because no one can ethically assign a teenager to use cannabis. Everything below comes from observing young people who chose to use it and comparing them with those who did not, which means other differences between those groups are always part of the picture.
IQ and school. A New Zealand study that followed people from birth found that persistent cannabis dependence was associated with a loss of up to six IQ points by age 38, with the effect strongest among those who became dependent before 18. That finding, described in a 2020 PubMed Central article, is the one most often quoted. Less often quoted is what the same research group published in 2018. Tracking twins in England and Wales from age 5 to 18, they concluded that cannabis was not the cause of the IQ decline, and that family background factors likely explained why the teenagers who used cannabis scored worse.
Brain scans. A study compared 43 healthy adolescents with 22 adolescents in treatment for cannabis use disorder. Over time, the healthy group gained connectivity between two regions at the front of the brain while the cannabis group did not, and lost it elsewhere. Heavy use across the following 18 months predicted lower IQ and slower thinking at follow-up. The same study found the relationship running the other way as well: teens with weaker connectivity at the start went on to use more cannabis, which suggests the influence goes in both directions.
Reviewing the imaging research more broadly, Blest-Hopley and colleagues report that adolescent cannabis users show different patterns of brain activity from non-users, most often more activity rather than less. Their reading is that the brain may be working harder to get the same result. They also note reduced white matter integrity, meaning the quality of the brain’s wiring, in adults who started using in early adolescence.
What happens when people stop. The same 2020 PubMed Central article describes research at Massachusetts General Hospital in which regular users aged 16 to 25 who stopped for four weeks saw improved verbal memory, with most of the gain in the first week. One of that study’s authors put the limits plainly: they do not know how long it would take those users to reach the level of a non-user, or whether they would, only that “continued use is worse than stopping.” Beyond that the picture is unsettled. Blest-Hopley and colleagues note that some brain differences appear to normalize after a period without cannabis while others persist, and that when researchers combined the results of many studies on thinking skills, the overall difference between users and non-users was small, and it did not hold up at all in the studies that looked only at people who had stopped using.
Cannabis and the Risk of Psychosis
Of everything studied about teenage cannabis use, the link to psychosis and schizophrenia has drawn the most attention, and the most argument.
The association itself is well documented. A 1987 study of more than 45,000 Swedish men, examined when they entered mandatory military service at around age 19 and again 15 years later, found that those who had used cannabis more than 50 times by that first exam were six times more likely to be diagnosed with schizophrenia by the 15-year mark. The association was weaker, though still present, after accounting for difficult childhood circumstances and other diagnoses. A 2002 analysis of roughly 760 New Zealanders followed since birth found that starting cannabis by age 15 was associated with a fourfold higher risk of developing schizophrenia by age 26, while starting closer to age 18 carried only a small increase that was not statistically meaningful. Both are described in a 2020 PubMed Central article on cannabis and the adolescent brain.
The pattern runs in a consistent direction: earlier, heavier, and stronger. The CDC notes that the association between cannabis and schizophrenia is stronger in people who start using at an earlier age and use more frequently, and StatPearls notes that high-THC exposure increases the risk of psychosis.
What is not settled is what causes what. Researchers broadly agree a relationship exists; they disagree about which direction it runs. Some people may be drawn to cannabis partly because of an existing vulnerability. One study of more than 2,000 adults found that people carrying gene variants linked to schizophrenia risk were more likely to use cannabis, and to use more of it. There is also a puzzle in the population numbers: cannabis use in the Western world has risen dramatically since the 1960s, while rates of schizophrenia, often cited at around 1% or less, have not changed much over the same period. Many researchers land in the middle, holding that cannabis triggers or speeds up psychosis in people already predisposed to it rather than producing the illness on its own.
Animal research offers one possible explanation for how that could work. In rats, THC exposure during adolescence disrupted the signaling that normally keeps activity in the front of the brain in check, which in turn left dopamine cells overactive in a region tied to motivation and reward. The researcher behind that work notes that similar features have been observed in the brains of people with schizophrenia.
For a parent or a young person, the practical picture does not really depend on which theory wins. The risk sits in the same place either way: starting young, using often, and using strong products.
What It Means for the Rest of Life
The effects that show up on brain scans matter mostly because of what they connect to. Three things come up repeatedly in the research.
School and work. The CDC reports that teens who use cannabis are more likely to quit high school or not finish a college degree. Fischer and colleagues describe the same pattern extending further out, with a greater likelihood of dropping out of school and weaker long-term academic and work achievement.
Addiction. About 3 in 10 people who use cannabis have cannabis use disorder, according to CDC, and the risk is higher for people who start young and use more often. StatPearls puts a number on the age effect: starting before 15 carries roughly double the lifetime risk. As of 2023, the highest rate of cannabis use disorder in the US was among people aged 18 to 25, at 16.6%.
Driving. Cannabis affects reaction time, coordination, and concentration, all of which driving depends on. Fischer and colleagues list impaired driving ability alongside impaired thinking and sleep among the effects of adolescent exposure.
Some of this reflects differences that already existed between teenagers who use cannabis and those who do not, and the research cannot fully separate the two. But the pattern is consistent enough across studies that it is worth taking seriously on its own terms.
Higher Potency and a Changing Risk Picture
Most of the research described above studied a weaker drug than the one a teenager can buy today. Fischer and colleagues note that studies measuring THC’s effects have generally used cannabis supplied by the National Institute on Drug Abuse containing roughly 3% to 5% THC. In states with legal recreational and medical cannabis, products now average 16% to 35% THC, and extract products run upwards of 60%.
The strength appears to change the risk, not just the experience. Fischer and colleagues report that higher-THC preparations carry a dose-dependent increase in the risk of developing a substance use disorder, meaning the more concentrated the product, the higher the risk. In a study of more than 400 people experiencing a first episode of psychosis, three things, starting cannabis at 15 or younger, using daily, and using higher-THC products, moved that first episode earlier by two to six years.
At the same time, teenagers see cannabis as less risky than they used to. The Monitoring the Future survey, which polls about 50,000 American adolescents every year, has recorded a substantial decline in perceived risk from regular cannabis use over the past two decades, alongside rising use. Vaping is part of that shift: roughly 1 in 10 adolescents report vaping cannabis, and teens tend to view vaping extracts as safer than smoking.
Whether legalization itself drives teen use is less clear. A 2020 PubMed Central article describes early surveys reaching different conclusions, and points to one study of Colorado, Washington, Alaska, and Oregon that found no change in how many adolescents used cannabis after legalization, but did find a slight rise in cannabis use disorder among teens. Fischer and colleagues report that states which legalized have seen a 25% increase in problematic cannabis use among adolescents compared with states that have not. The two findings fit together: the share of teenagers using may not move much, while the share running into trouble does.
Where This Leaves Parents and Teens
The point of everything above is not that cannabis ruins lives, and not that it is harmless. It is that timing matters. A brain that is still being built responds differently than a finished one, and those building years overlap with the years when many young people first try cannabis.
That gives parents something more useful than a warning. Waiting is worth arguing for, and later really is better than earlier. It is also a message a teenager can hear without dismissing it, because it does not overstate the case.
If a young person is already using, the conversation is worth having sooner rather than later, and not as an accusation. Ask how things are going at school, at home, and with mood. Those are usually where a problem shows up first, and they are easier to talk about than the substance itself.
If cannabis is causing trouble in any of them, a physician, a counselor, or a treatment or community program can help work out what is happening and what would help. Cannabis use disorder is treatable, and it is easier to address early than late.