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Marijuana and the Developing Brain: The Science in 2019

The fear of the teen brain on drugs has plagued parents for generations. Sensational anti-marijuana ads have aired on television sets, and print ads have been circulating since the 1930s. The now infamous 1936 film Reefer Madness called it “the burning weed with its roots in hell.” However, despite the efforts to scare kids away from marijuana, it remains one of the most popular drugs in the world.

A 2017 survey found that more than 14 percent of people said they used cannabis in the past year. In 2016, more than 9 percent of eighth-graders reported using marijuana in the past year.

Though the scare tactics of the 1930s were backed by very little scientific evidence, the facts may be scary enough on their own. Plus, today’s illicit marijuana is a lot more powerful than the days of reefer madness. Marijuana contains dozens of active cannabinoids, but the two most abundant are cannabidiol (CBD) and tetrahydrocannabinol (THC). Both of these chemicals bind to one specific receptor (CB2) and interact differently with another receptor (CB1), but they have different effects on the body.

THC causes potent psychoactive effects while CBD does very little, and may even counteract some of the effects of THC. For that reason, illicit drug manufacturers create strains of marijuana with high amounts of THC and low amounts of CBD to increase potency. Powerful marijuana allows them to make a higher profit on smaller and easier to transport shipments of the drug.

On the other side of the argument, people promote the medical benefits of marijuana and point out the ways that it’s much less harmful than legal drugs like alcohol and nicotine. But what’s the truth behind the staunch opposition and unwavering advocacy, especially when it comes to the teens that are taking the drug? Here’s some of the most recent science about marijuana and brain development.

Does Marijuana Affect Cognitive Development?

According to a 2015 review, marijuana does seem to have a clear effect on cognition in adolescence, but we aren’t sure if that’s caused by residual effects of marijuana in a person’s system, or if it’s caused by developmental issues. The review says, “…teens who engage in heavy marijuana use often show disadvantages in neurocognitive performance, macrostructural and microstructural brain development, and alterations in brain functioning.”

Though that seems like a smoking gun, the review goes on to say, “It remains unclear whether such disadvantages reflect pre-existing differences that lead to increased substances use and further changes in brain architecture and behavioral outcomes.”

Proponents of marijuana use often point out that marijuana is much less harmful than alcohol in many ways. For the most part, that seems to be true when it comes to the development of diseases like cancer, liver disease, and cardiovascular disease.

A 3D image of a brain at risk of harm from marijuana

However, a recent 2018 study published in the American Journal of Psychology has shown that marijuana may be just as, if not more, dangerous as alcohol when it comes to cognition and neurotoxicity. The study looked at a sample of more than 3,800 seventh-graders in Montreal and investigated the effects of both marijuana and alcohol use on a variety of cognitive functions.

The study concluded by saying, “…the concurrent and lasting effects of adolescent cannabis use can be observed on important cognitive functions and appear to be more pronounced than those observed for alcohol.” Marijuana has been shown to have neurotoxic effects on inhibitory control (impulse control) and working memory. It also affected delayed memory recall and perceptual reasoning.

Another study that was published in 2018 followed up with seven men throughout a 40-year period, starting at age 8. The study found an association between marijuana use before the age of 18 and the development of major depressive disorder. Though this doesn’t definitively prove that marijuana use during development causes depression later in life, it does point to marijuana as a potential developmental risk factor for depression.

While both of these studies only look at a relatively small number of marijuana users compared to the overall drug-using population, it does show that the dismissal of marijuana’s harms might be a mistake. Most marijuana advocates don’t advocate for its use in adolescents, but it’s important that the language surrounding marijuana is accurate about its potential harms and benefits.

Can Marijuana Cause Psychosis?

Marijuana has different psychoactive effects in different people, but in some cases, it’s reported to lead to psychotic symptoms. However, it’s difficult to determine whether the drug causes psychosis, or if it triggers latent psychotic disorders. Still, marijuana use is associated with psychotic disorders like schizophrenia, especially when it comes to high-potency marijuana.

Marijuana plants in a dimly lit room

THC can cause powerful psychoactive effects that may lead to psychosis. However, CBD serves to counteract these effects, helping to prevent psychotic episodes. But since CBD levels are lowered in high-potency marijuana, it has a greater chance of causing psychotic effects. According to NPR, researchers found that episodes of psychosis were higher in European cities where high-potency marijuana was readily available. The study also found that people who used marijuana every day were three times more likely to experience symptoms of psychosis that people who don’t use it.

A University of Mississippi study found that the average concentration of THC in marijuana was a little more than 17 percent in 2017, which is up from 8.9 percent a decade earlier.

Is Marijuana a Gateway Drug?

Marijuana has a reputation as a gateway drug, which is a drug that leads to the use of harder substances. A 1998 study argued that marijuana could lead to more illicit drug use by positive reinforcement or by its complementary effects on other drugs like cocaine. It’s true that there is a significant link between marijuana and harder drug use.

A 2014 study found that more than 44 percent of individuals that used cannabis over their lifetime progressed to other illicit drugs. But it’s possible that other factors are involved. For instance, it may be that the same factors that cause a person to try marijuana contribute to their willingness to try other illicit drugs.

People who use illicit drugs also may start by drinking alcohol or smoking cigarettes, which could also be risk factors that lead to illicit drug use later in life.

The U.S. Centers for Disease Control and Prevention (CDC) has acknowledged that the gateway drug theory for marijuana lacks evidence. They say, “The majority of people who use marijuana do not go on to use other, ‘harder’ substances. More research is needed to understand if marijuana is a ‘gateway drug.’”


The American Journal of Psychiatry. (2018, October 3). A Population-Based Analysis of the Relationship Between Substance Use and Adolescent Cognitive Development. from

Carroll, L. (2018, August 27). One in seven U.S. adults used marijuana in 2017. from

Chandra, S., Radwan, M. M., Majumdar, C. G., Church, J. C., Freeman, T. P., & ElSohly, M. A. (2019, February). New trends in cannabis potency in USA and Europe during the last decade (2008-2017). from

Chatterjee, R. (2019, March 19). Daily Marijuana Use And Highly Potent Weed Linked To Psychosis. from

DeSimone, J. (1998). Is Marijuana a Gateway Drug? Eastern Economic Journal, 24(2), 149-164. from

Di Forte, M., Ph.D., Quattrone, D., M.D., Freeman, T. P., Ph.D., Tripoli, G., MSc, Gayer-Anderson, C., Ph.D., & Quigley, H., M.D. (2019, May 01). The contribution of cannabis use to variation in the … from

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Schoeler, T., Theobald, D., Pingault, J., Farrington, D. P., Coid, J. W., & Bhattacharyya, S. (2018, October). Developmental sensitivity to cannabis use patterns and risk for major depressive disorder in mid-life: Findings from 40 years of follow-up. from

Secades-Villa, R., Garcia-Rodriguez, O., Jin, C. J., Wang, S., & Blanco, C. (2014, August 02). Probability and predictors of the cannabis gateway effect: A national study. from

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