Every now and then, a recent marijuana study will make headlines on its detrimental effects on the brain. There’s one that says marijuana increases the risk for schizophrenia, while a newer study shows that weed use does not really shrink brain size among teens. The conflicting results indicate just how much research is still needed in order to paint a complete picture on the real effects of marijuana on the brain. After all, it’s not just cannabis that plays a role in brain development—genes, culture, and environment do, too.
On Genes and Environment
Geneticist Arpana Agrawal from the Washington University in St. Louis studied the MRI brain scans of 241 pairs of same-sex siblings, including twins, to check how much weed has altered their brains. In some siblings, only one has smoked marijuana, while in others, both have smoked pot. There are also siblings who haven’t tried cannabis even once.
Agrawal’s team found out that in general, teens who have smoked pot even once have smaller brain volume in the amygdala—a region associated with emotion processing and reward seeking—and in the right ventral striatum—also associated with reward processing—compared to those who haven’t tried smoking marijuana.
On the other hand, the siblings who did not smoke pot but shared similar genes and environment with their weed-smoking siblings also had smaller volumes in these brain areas compared to siblings who both avoided smoking marijuana.
“This suggests that there might be common factors, genetic and environmental, that predispose us to using marijuana that also contribute to variations in our brain volumes,” Agrawal said in an interview with Live Science.
On Marijuana and Schizophrenia
In another study, Dr. Tomas Paus from the Rotman Research Institute in Toronto studied the MRI brain scans of more than 1,500 teenage boys to see if smoking marijuana had any effect on their risk for developing schizophrenia.
Paus’ team found out that over the course of four years, those who had smoked pot and had predispositions to develop schizophrenia had a thinning of the cortex—the brain’s outside covering—compared to those who had the same genes but didn’t smoke pot.
Though it still isn’t clear what the thinning of the cortex means, it does point out to the possibility that marijuana is the primary cause of this problem. After all, the brain regions that had the most cortical thinning also happened to have the highest concentrations of cannabinoid receptors.
For Paus, this study does not provide conclusive evidence that marijuana really causes schizophrenia. “All we are saying is that if you combine cannabis use with the genetic risk, then the brain is maturing in a slightly different way,” he said.
While scientists are still trying to figure out what marijuana’s effects on the brains are, Dr. David Goldman of the National Institute on Alcohol Abuse and Alcoholism in Maryland wrote that the effects of weed may vary among people due to diversity in genotype and environment, and even the dose of cannabis given. More studies will definitely be needed in order to fully understand the impact of marijuana on young brains.
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