What does weed do to the teenage brain

Smoking pot as a teen likely affects the still-developing brain, a new study finds. The region most at risk helps guide decision-making.

Matthew Albaugh is a psychologist at the University of Vermont in Burlington. He was part of a team that scanned teens’ brains with an MRI machine before and after they started to use marijuana (also called cannabis or pot). The study included 799 teens in Germany, France, Ireland and England.

Participants had their first scan at age 14. None reported using marijuana at this point. Five years later, the teens returned for a second scan. Now 369 of the adolescents (46 percent) reported they had tried cannabis. About three-quarters of these said they had done so at least 10 times.

One part of the brain changed more in cannabis users than non-users. Called the prefrontal cortex, it sits right behind the forehead and above the eyes. This region is involved in decision-making and other tasks. It tends to thin during adolescence to help the brain work more efficiently. But that thinning sped up in teens who reported cannabis use. The more drug the teens used, the faster the prefrontal cortex thinned, Albaugh’s team now reports.

That may sound like a good thing, like the brain is maturing more quickly on pot. But researchers don’t view it that way. Notes Albaugh, exposing young animals to pot causes their brain to thin too early. That can lead to long-lasting problems with behavior and memory.

The teen study does not prove that cannabis caused the faster thinning. But it adds to growing evidence that early cannabis use may affect brain development.

Albaugh’s group described its findings June 16 in JAMA Psychiatry.

Brain ‘pruning’ and cannabis

Jacqueline-Marie Ferland is a brain researcher at the Icahn School of Medicine in New York City. The prefrontal cortex is “the adult in the room,” she says. One of its jobs is to combine different pieces of information as we make decisions. A mature prefrontal cortex, she says, controls behavior by toning down emotions. It also tamps down impulsive actions.

Teens often take more risks than children and adults. The reason? Different parts of the teen brain mature at different rates. Greater risk-taking often promises greater rewards. The prefrontal cortex that guides rational behavior matures more slowly than do parts of the brain that control our emotional response to rewards. In fact, the prefrontal cortex is the last brain region to fully mature. It’s not complete until around age 25. Thinning is an important part of that process.

Janna Cousijn is a psychologist at Erasmus University in Rotterdam, the Netherlands. She compares a young child’s brain to a very thick forest. “As we grow up, we often decide to take similar paths within that forest,” she says. That means some frequently traveled paths — connections among brain cells — start to emerge.

Favored paths become better established as we age. This lets brain signals pass down them faster. Rarely or never-used paths disappear. Thinning of the prefrontal cortex is part of this “pruning.”

The active chemical in cannabis is called THC. It speeds up thinning of a rat’s version of the prefrontal cortex. THC binds to docking stations in brain cells. Those docking stations are called CB1 receptors. That CB is short for cannabinoid (Kah-NAA-bin-oid), meaning these receptors are ideally structured to respond to cannabis compounds known as cannabinoids. These include THC.

Rats given THC during adolescence lose some brain connections that would otherwise stick around into adulthood. This can alter the rodents’ behavior and memory. How much depends on the amount of THC and the animal’s age.

Researchers cannot study a direct link between THC and prefrontal-cortex thinning in people. But Albaugh’s colleagues found more CB1 receptors, on average, in the prefrontal cortex of 21 adult men than in other brain regions. (The imaging method uses a radioactive material, so this could not ethically be confirmed in teens.)

It is notable that the CB1-rich part in adult brains overlaps with the area that thins faster in cannabis-using teens. The overlap does not prove that cannabis causes the change. It does, however, add to evidence that it may.

A window of vulnerability

Cannabis has medical benefits for some people. Adults can legally use it in some U.S. states and countries. But that, says Albaugh, does not mean cannabis is harmless for young users. “Brain areas that change the most during adolescence may be especially vulnerable to cannabis exposure,” he says.

To learn if the thinning persists into adulthood, he is now analyzing the same people’s brain scans at age 23. He will also test if the brain changes his group has recorded relate to unwanted adult outcomes. This may include lower graduation rates, delayed graduation or more mental health disorders.

“It is critical to know if teen cannabis use may change how you function as an adult,” says Ferland. Until more is known, many researchers recommend postponing any cannabis use until adulthood. They also suggest limiting its frequency and using only low-potency products.

One last thing to note: Alcohol is the most common drug of adolescence in many countries. And researchers have found that alcohol and cigarettes often do more damage to the brain than cannabis. (The amount used matters greatly for all three.) But even smaller brain changes from regular use can lead to addiction. “And developing any addiction is damaging to yourself and others,” says Cousijn. Adds Ferland, “starting to use drugs as a teen greatly increases the risk of addiction later in life.”

American parents have been warning teenagers about the dangers of marijuana for about 100 years. Teenagers have been ignoring them for just as long. As I write this, a couple of kids are smoking weed in the woods just yards from my office window and about a block and a half from the local high school. They started in around 9 A.M., just in time for class.

Exaggerating the perils of cannabis—the risks of brain damage, addiction, psychosis—has not helped. Any whiff of Reefer Madness hyperbole is perfectly calibrated to trigger an adolescent's instinctive skepticism for whatever an adult suggests. And the unvarnished facts are scary enough.

We know that being high impairs attention, memory and learning. Some of today's stronger varieties can make you physically ill and delusional. But whether marijuana can cause lasting damage to the brain is less clear.

A slew of studies in adults have found that nonusers beat chronic weed smokers on tests of attention, memory, motor skills and verbal abilities, but some of this might be the result of lingering traces of cannabis in the body of users or withdrawal effects from abstaining while taking part in a study. In one hopeful finding, a 2012 meta-analysis found that in 13 studies in which participants had laid off weed for 25 days or more, their performance on cognitive tests did not differ significantly from that of nonusers.

But scientists are less sanguine about teenage tokers. During adolescence the brain matures in several ways believed to make it more efficient and to strengthen executive functions such as emotional self-control. Various lines of research suggest that cannabis use could disrupt such processes.

For one thing, recent studies show that cannabinoids manufactured by our own nerve cells play a crucial role in wiring the brain, both prenatally and during adolescence. Throughout life they regulate appetite, sleep, emotion, memory and movement—which makes sense when you consider the effects of marijuana. There are “huge changes” in the concentration of these endocannabinoids during the teenage years, according to neurologist Yasmin Hurd of the Icahn School of Medicine at Mount Sinai, which is why she and others who study this system worry about the impact of casually dosing it with weed.

Brain-imaging studies reinforce this concern. A number of smallish studies have seen differences in the brains of habitual weed smokers, including altered connectivity between the hemispheres, inefficient cognitive processing in adolescent users, and a smaller amygdala and hippocampus—structures involved in emotional regulation and memory, respectively.

More evidence comes from research in animals. Rats given THC, the chemical that puts the high in marijuana, show persistent cognitive difficulties if exposed around the time of puberty—but not if they are exposed as adults.

But the case for permanent damage is not airtight. Studies in rats tend to use much higher doses of THC than even a committed pothead would absorb, and rodent adolescence is just a couple of weeks long—nothing like ours. With brain-imaging studies, the samples are small, and the causality is uncertain. It is particularly hard to untangle factors such as childhood poverty, abuse and neglect, which also make their mark on brain anatomy and which correlate with more substance abuse, notes Nora Volkow, director of the National Institute on Drug Abuse and lead author of a superb 2016 review of cannabis research in JAMA Psychiatry.

To really sort this out, we need to look at kids from childhood to early adulthood. The Adolescent Brain Cognitive Development study, now under way at the National Institutes of Health, should fill the gap. The 10-year project will follow 10,000 children from age nine or 10, soaking up information from brain scans, genetic and psychological tests, academic records and surveys. Among other things, it should help pin down the complex role marijuana seems to play in triggering schizophrenia in some people.

But even if it turns out that weed does not pose a direct danger for most teens, it's hardly benign. If, like those kids outside my window, you frequently show up high in class, you will likely miss the intellectual and social stimulation to which the adolescent brain is perfectly tuned. This is the period, Volkow notes, “for maximizing our capacity to navigate complex situations,” literally building brainpower. On average, adolescents who partake heavily wind up achieving less in life and are unhappier. And those are things a teenager might care about.