One time use of Marijuana could affect teen brains finds study

A survey from 2018 had found that 2.1 million students from middle and high schools have smoked marijuana at least once. A new study looked at the effects of marijuana or cannabis on the brains of adolescents.

The results of the study were published in the latest issue of the Journal of Neuroscience this week. The researchers were alarmed to find that marijuana or cannabis can affect the teenage brains after even a single use.

The team of researchers looked at the brains of 46 fourteen year old students from England, Ireland, France and Germany. They noted that those of the participants who had smoked just one or two joints had a higher brain volume compared to the teenagers that did not. The gray matter in these teenagers was larger. This area of the brain is altered with maturity of the brain. The National Institutes of Health (NIH) says that with aging the brain size normally decreases. The researchers have said that this study shows that marijuana could affect the normal aging process of the brain.

Hugh Garavan, lead author of the study and a professor of psychiatry at the University of Vermont School of Medicine, explained that the cortical region of the brain of these teenagers were “going through a process of thinning.” He said that the normal maturation and pruning process of the brain was being affected by the marijuana. “So, one possibility is that the cannabis use has disrupted this pruning process, resulting in larger volumes (i.e., a disruption of typical maturation) in the cannabis users. Another possibility is that the cannabis use has led to a growth in neurons and in the connections between them,” he said.

Garavan said in a statement, “Most people would likely assume that one or two uses (joints) would have no impact, so we were curious to study this — and especially to investigate if first uses may actually produce brain changes that affect future behavior like subsequent use.”

The team accepts that this is a small study with a small number of participants and thus the results should be interpreted carefully. Larger and long term studies are needed to understand the effects of marijuana on the brain and health. Marijuana contains tetrahydrocannabinol, or THC, a psychoactive component that leads to several problems including slowing of the reaction time, raising heart rates and also affects the short term memory of the individual.

“As is always the case, more research is needed to replicate these effects, to try to understand the mechanisms, and critically, to unearth what additional factors may identify which cannabis-using kids show these effects and which ones don’t,” said Garavan.

Data from the National Institute of Drug Abuse has said that 32.6 percent of the tenth graders have used marijuana at least once. On the flip side, 10 states in the United States and the District of Columbia have legalized the recreational use of marijuana. It is also being studied in disease conditions such as pain, muscle spasms, seizures, nausea seen with cancer chemotherapy etc.

Dr. Ananya Mandal

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.


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  1. Devon Wallace Devon Wallace United States says:

    "You’re changing your brain with just one or two joints"

    This is very unlikely. More likely, this very small study (46 teens), could have found a link of why some teens choose to use cannabis while others do not (reverse causation). The researchers don't even know the significance of this size increase. Regardless, it doesn't mean much without at least being replicated by larger studies.

    Teen usage did not rise when Colorado and Washington State legalized.

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
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