According to The National Institute on Drug Abuse (NIDA) for Teens, marijuana is the most popular illicit drug for teens (and adults). Teens are definitely using marijuana and no it’s not because of legalization. You’ve probably heard that using weed in the adolescent years can cause certain problems for people later on in life. Do we know if weed causes a change in the adolescent brain that’s a real cause for concern? We can speculate forever, let’s see if has science found a link between adolescent weed use and adult brain structure.

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Does Adolescent Weed Use Affect Adult Brain Structure 

According to a study published in Drug and Alcohol Dependence journal, adolescent cannabis use is not associated with structural brain differences in adulthood. This was a longitudinal study of 1000 boys who were tasked with completing self-reports about their cannabis use from ages 13-19. The researchers used latent class growth analysis to identify different trajectories of adolescent cannabis use. The groups were as follows: non-users/infrequent users, desisters, escalators, and chronic-relatively frequent users. The boys were subsequently placed in their most likely cannabis trajectory.

A subset of 181 from the group was chosen for structural neuroimaging. This subset, on average, was between ages 30-36. The men were grouped by their previous cannabis trajectory. The researchers then investigated whether there were any differences in adult brain structure in 14 a priori regions of interest, including six subcortical and eight cortical regions.

According to the study, “Boys in different trajectory subgroups did not differ on adult brain structure in any subcortical or cortical region of interest.” So in conclusion, using weed as a teenager is not associated with structural brain differences in adulthood, according to this study. 

Other Studies Examining Adolescent Marijuana Use and Brain Structure 

Several other studies have examined adolescent marijuana use and did find that the brain structure is affected. 

In a study titled Neurocognitive Correlates of White Matter Quality in Adolescent Substance Users, the researchers found that in cannabis users, lower fractional anisotropy (FA) in temporal areas related to poorer performance on attention, working memory, and speeded processing tasks. The researchers tested 36 chronic marijuana and alcohol-using adolescents and 36 non-users as a control group. They sought to examine the cognitive manifestations of altered white matter microstructure in adolescents. From our understanding, this study looked at teens who used both alcohol and marijuana. Alcohol use could account for differing results from the previous study.

Another study also found that teenaged cannabis users between the ages of 16-19 had decreased right medial orbital prefrontal cortex volume compared to their counterparts who weren’t using cannabis. 

A 2011 study found that marijuana use may have an effect on the development of the hippocampus region. According to the study, “Heavy-cannabis users showed significantly smaller volumes of the right and left  hippocampus, but no significant differences in the amygdala region compared to controls”

White matter integrity seems to be poorer in adolescents who use marijuana in comparison to those who do not. According to the Effects of Cannabis on the Adolescent Brain, “In two studies published in 2008 and 2009, we found poorer white matter integrity (e.g., decreased FA and increased MD) in several association and projection fiber tracts in adolescent cannabis users with concomitant alcohol use.”

Even with all these studies, according to Effects of Cannabis on the Adolescent Brain “large longitudinal research would also help clarify the degree to which pre-existing differences and/or chronic marijuana use during adolescence contributes to the development of psychiatric disorders and cognitive impairment in adulthood”. 

Other Effects of Marijuana on Teen Brains

There have been other studies that have shown using cannabis during adolescence affects the brain. According to an article titled, Effects of Cannabis on the Adolescent Brain, cannabis use is associated with attention issues, memory problems, possible macrostructural brain alterations, changes in white matter tract integrity and abnormalities of neural functioning.  

The authors of Effects of Cannabis on the Adolescent Brain, found in their own study that cumulative marijuana use over the course of eight years was related to poorer performance on measures of attentional functioning. In this study, they followed teens between 13 to 30. The article said in a subsequent study, teens between the ages of 16-18 who used marijuana “demonstrated slower processing speed, poorer verbal learning and memory, and sequencing abilities”. 

Using marijuana as a teen can also increase risk of dependency. According to the National Institute on Drug Abuse, people who use marijuana before the age of 18 are four to seven times more likely to develop a marijuana use disorder than adults. In addition to that, Kevin Sabet, PhD, president of Smart Approaches to Marijuana was quoted in Healthline saying that marijuana use can harm the developing brain, increase the risk of severe mental illness, and may even lead to future substance abuse.

Is Marijuana’s Effects on the Adolescent Brain Long Lasting?

What was interesting is that the same researchers who conducted the eight-year study aforementioned decided to see if abstinence from weed differed. So, they conducted a separate study that examined neurocognitive performance over a period of 3 weeks of monitored abstinence in teenaged marijuana users ages 15–19. According to the study, data were collected from 65 adolescent marijuana users and controls between the ages of 16 to 18 years old. The researchers found out that adolescent cannabis users demonstrated slower psychomotor speed,  poorer complex attention, story memory and planning and sequencing ability compared with the teens that didn’t use weed. The researchers believe that based on the general pattern of results even after a month of monitored abstinence, teens who use weed will have subtle neuropsychological deficits compared with nonusers. It was then concluded that “It is possible that frequent marijuana use during adolescence may negatively influence neuromaturation and cognitive development.”

In another study that looked at young people between ages 20-24, researchers found that even though the participants had memory deficits, they showed improvements after eight years of not using marijuana. This shows that maybe the effects of cannabis on the brain aren’t permanent or could be reversed. 

Is Weed Better Than Alcohol For Teen Brains?

Not according to a study called A Population-Based Analysis of the Relationship Between Substance Use and Adolescent Cognitive Development, the effects of weed on teen’s brains may be more pronounced that alcohol. The study said, “Cannabis use, but not alcohol consumption, showed lagged (neurotoxic) effects on inhibitory control and working memory and concurrent effects on delayed memory recall and perceptual reasoning (with some evidence of developmental sensitivity)”. The study also stated that cannabis use in any given year impaired inhibitory control and working memory 1 year later. 

Danielle Ramo, PhD, associate professor in residence and licensed psychologist was quoted in Healthline saying that cannabis (and alcohol) affects the adolescent brain by impacting the memory and executive functioning. She also said that continuous use of cannabis throughout adolescence leads to a worsening of the impact especially if the individual started earlier. Ramo said “Earlier use that persists throughout adolescence is associated with greater burden on teens’ ability to process new information and to ‘stop and think’ in the face of complex stimuli,” Ramo continued. “These effects may even be greater than the effects of alcohol on the teen brain.”

However, in another study, cited in Effects of Cannabis on the Adolescent Brain, teens who binge drink had worse white matter integrity that those who were cannbis users. The article said “teens engaging in binge drinking only looked significantly worse on indices of white matter integrity (i.e., decreased FA) in several areas (cortical and subcortical projection fibers) as compared to marijuana users. The researchers believed that this piece of information highlights “the need for further research to disentangle the effects of marijuana and alcohol on the developing brain”

Conclusion

It’s a good thing that there are many studies that look at the effects of cannabis on adolescent brains. One study said that adolescent weed use didn’t have an impact on adult brain structure. However, many other studies show the marijuana does change the brain structure of teens as well as affect their brains in different ways. There still needs to be more studies done to see how the changes highlighted in the article affect teen users as adults. With that said, it seems clear that using weed as an adolescent does have an effect on the brain with measurable impacts. The experts seem to discourage the use of weed for teens for those reasons.

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