Marijuana: Effects on the Brain

Marijuana is considered a Schedule I drug by the DEA, meaning that marijuana is a drug with “no currently accepted medical use and a high potential for abuse” (DEA 2018). There have been 32 states in the United States that have legalized marijuana for medical use, while 11 states have legalized marijuana for recreational use. Due to this classification by the DEA and federal government, medical testing of marijuana has been highly limited to animal testing and volunteer human trials. With the newly found interest in the affects of marijuana on humans for medical purposes, new information continues to surface regarding marijuana’s effect on the human brain.

Marijuana has been documented to have short- and long-term effects on the human brain. The human brain contains receptors that react to naturally occurring THC (tetrahydrocannabinol)-like chemicals that aid in the brain’s development and function. When marijuana is consumed, regardless of whether it is inhaled or absorbed, it causes these THC receptors to overreact. This overreaction leads to the short-term effect known as the “high”. The short term effects of the high can include: altered senses (seeing brighter colors), altered sense of time, changes in mood, impaired body movement, difficulty in critical thinking and problem solving, and impaired memory. When consumed in high doses a person could experience hallucinations, delusions, and psychosis (National Institute of Drug Abuse 2018).

The short term effects tend to be limited to the “high” that occurs after consuming marijuana, but many scientists have studied the long term effects of marijuana on the human brain. Trinity College of Neuroscience conducted a study into the long-term effects of marijuana on neonatal, pubertal, and adult brains. Marijuana is the most widely used drug by pregnant women and has effects on the fetus and later infant. The abuse of this drug during pregnancy was “associated with increased tremors and startles in infants and poorer habituation to visual stimuli” (Downer 94, Campbell 94). Although this study lacked to find a correlation between prenatal marijuana use and early childhood development, specifically between the ages of one and three years old.

Other studies showed that impairments in children at the age of two and three included short-term memory, verbal and abstract reasoning due to their mothers consuming marijuana. Over all, children showed hyperactivity and increased prevalence of childhood depression by the time they were ten years old. Although the evidence so far has not been able to fully conclude the purely negative side effects of marijuana exposure on the fetus brain, a new study using MRI imaging on the brains of children exposed to drugs while in the womb, specifically marijuana, showed a reduction of cortical gray matter and parenchymal volume in children from the age of ten to fourteen. This and other studies are beginning to show the predominantly negative effects of marijuana exposure during pregnancy (Downer 94, Campbell 94).

When looking at adolescent exposure to marijuana, it has been proven that it can have damaging effects to the developing mind. A study done by Natasha E. Wright, Danny Scerpella, and Krista M. Lisdahl looked to prove that marijuana usage in adolescents and young adults correlate with depressive symptoms and behavioral approach. The study contained 42 marijuana users and 42 members of the controlled group. They concluded that the marijuana group showed increased depressive symptoms, decreased fun-seeking, and a decreased reward response. Female users showed an increase in anxiety like symptoms. This study concluded that regular marijuana use needs to be prevented in adolescents and young adults (Wright 2016, Scerpella 2016, Lisdahl 2016).

Yet another study addressed the same issue of adolescent use of marijuana. They focused on heavy, regular marijuana users who began before and after the age of 16. The study showed that compared to users who started after 16, the early smokers made “twice as many mistakes on tests of executive function, which included planning, flexibility, abstract thinking and inhibition of inappropriate responses. Not only where their basic functioning affected, early smokers as adults were found smoking 25 times per week, while their counterparts smoked about half as much. Also, the early smokers consumed 15 grams a week compared to six grams by the later smokers. This study went on to find that these smokers also had changes in the shape of their brain. Through MRI testing they were able to notice that the volume and gray matter density of two brain regions associated with addiction. These parts of the brain are known as the nucleus accumbens, the part of the brain that controls motivations, pleasure, and reward processing, while the other is the amygdala, the region involved in memory, emotion, and decision-making (Weir 2015).

There are positive aspects of using marijuana. There is evidence that it increases the connectivity of the brain compared to people who do not smoke marijuana. Dr. Sina Aslan, Founder and President of Advance MRI and Adjunct Assistant Professor at the University of Texas at Dallas said, “The results suggest increases in connectivity, both structural and functional, that may be compensating for gray matter losses. Eventually, however, the structural connectivity or ‘wiring’ of the brain starts degrading with prolonged marijuana use.” This increased connectivity is the greatest when users first begin using, especially in those who used more frequently than others. After prolonged use, six to eight years, increased structural wiring declines but still marijuana users continue to display more connectivity than non-users. This would explain why chronic marijuana users “seem to be doing just fine” despite the diminishing gray matter (Filloux 31).

Although THC is the most common aspect of marijuana, CBD (cannabidiol) is also found in marijuana. This component of the drug lacks the intoxicating properties that is found in its counterpart THC. CBD has become widely used to treat insomnia, anxiety, spasticity, and even life-threatening ailments like epilepsy (Grinspoon 2018). The health benefits of CBD have began to become widely used in the medical community as a way to naturally treat certain medical issues. A review of 19 patients by Dr. Francis M. Filloux showed that 50% of the group reported saying they had an 80% improvement in seizure frequency and severity. There still needs to be more human trials and testing done for CBD, but the FDA has granted orphan drug status to a drug called Epidiolex for the treatment of patients with Dravet and Lennox-Gastaut syndromes (Filloux 31).

Marijuana is still considered a Schedule I drug by the DEA and federal government, but 32 states have now legalized marijuana for medical use. With medical testing becoming more open to human trials for medical uses, time will tell whether more negative or positive side effects will come to the surface. As of the testing done up until this point, consuming marijuana strong in THC has been proven to hinder the growing brains of infants and adolescents while also affecting the gray matter of any age smoker.


  1. Downer, E. J., & Campbell, V. A. (2009). Phytocannabinoids, CNS cells and development: A dead issue? Drug and Alcohol Review,29(1), 91-98. doi:10.1111/j.1465-3362.2009.00102.x
  2. Drug Scheduling. (n.d.). Retrieved from
  3. Filloux, F. M. (2014). Cannabidiol Could Be a Valuable Treatment for Neuropsychiatric Disorders. Cannabidiol Could Be a Valuable Treatment for Neuropsychiatric Disorders,31. Retrieved November 27, 2018, from
  4. Grinspoon, P. (2018, January 09). Medical marijuana. Retrieved from
  5. National Institute on Drug Abuse. (n.d.). Marijuana. Retrieved from
  6. Weir, K. (2015, November). Marijuana and the developing brain. Retrieved from
  7. Wright, N. E., Scerpella, D., & Lisdahl, K. M. (2016, November 11). Marijuana Use Is Associated with Behavioral Approach and Depressive Symptoms in Adolescents and Emerging Adults. Retrieved November 27, 2018, from
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