Marijuana is a common psychoactive drug and is one of the leading drugs used by adolescents. The recent legalization of marijuana across many states in the US has only increased its use. In 2021, more than a third of high school seniors in the US reported using marijuana in the last year.
In the past, scientists thought marijuana could cause lung cancer since the most common method of marijuana use was smoking. But more recent scientists disproved this idea, since larger datasets showed people who use marijuana don’t have more lung cancer than the general population. Researchers are now suggesting marijuana could affect the brain more than the lungs.
Scientists think adolescents could be particularly vulnerable to brain damage from marijuana use, since their brains are still developing. The ability of the brain to change is called neuroplasticity. While the brain is developing, it is more easily changed. Brain development persists through adolescence, meaning drugs like marijuana have more potential to alter the brain during this time. Thus people who start using marijuana at a young age could experience long-lasting brain structural and behavioral changes, which impact their academic, social, and everyday lives.
An international team of researchers led by Matthew Albaugh recently performed a 5-year study of marijuana use in adolescents to test how it impacts their brain development. Their study was unique because it focused on humans, and most prior experiments were conducted on animals, like rats or mice. They also used direct brain imaging to see any long-lasting effects of marijuana use in these adolescents.
The participants included 799 adolescents between the ages of 12 and 15. Albaugh and colleagues asked the adolescents to self-report their use of marijuana, as well as other drugs like alcohol and tobacco, by filling out a questionnaire. They found some of the kids used marijuana, some used other drugs, and some did not use drugs at all.
The researchers imaged the adolescents’ brains at 2 different timepoints: once when they were middle schoolers, aged 12-15 years old, and again 5 years later, at 17-20 years old. They also measured how impulsive the participants were at the beginning and end of the 5-year study, using a 30-question self-reported survey called the Barratt Impulsiveness Scale.
The researchers explained one of the most notable brain changes that can occur with marijuana use is the thinning of the outermost layer of the brain, called the cerebral cortex. They used a type of imaging that uses magnetic waves, called magnetic resonance imaging, to directly view the thickness of the cerebral cortex in these adolescents.
The researchers found adolescents that used marijuana had more cerebral cortex thinning than those that did not. They also found the thickness of the cerebral cortex changed more in sites where marijuana acts in the brain, called cannabinoid-1 receptors. They knew where the cannabinoid-1 receptors were located from a brain map generated by other researchers.
The team found participants with a thinner cerebral cortex were also more impulsive after 5 years. They interpreted this correlation to mean the brain changes caused by marijuana use can also cause behavioral changes, like impulsivity.
The scientists concluded marijuana use can cause structural and behavioral changes in adolescents, but they noted several limitations to their study. One main limitation was that participants’ marijuana use was self-reported, so it could be inaccurate. The researchers also could not solely attribute the cerebral cortex thinning to marijuana use in these adolescents, since other mechanisms can cause similar changes in the brain. The team suggested future workers should conduct clinical trials with a controlled dosage of marijuana to determine if the amount of marijuana used can predict the amount of structural changes in the brain.
