Prohibitionists and fear-mongering media outlets often try to discourage popular support for cannabis reform by claiming that underage cannabis use can cause brain damage. Some of these claims were based on outdated research, but scientists are now discovering that alcohol, not weed, is responsible for these developmental issues.

A new study published in the Biological Psychiatry journal is adding to the growing evidence that alcohol impairs adolescent brain development in ways that weed does not. To conduct this study, researchers collected data on lifetime drug and alcohol use from 436 twins, all of whom were 24-years-old, and used MRI scans to determine the thickness of specific regions of each subject’s brain. 

In a study of unrelated subjects, it can be difficult to tell whether each individual’s unique genetic makeup or family upbringing is influencing the results of the study. But when twins are involved, researchers can compare two individuals with nearly identical genetics and family histories, which makes it easier to rule out these confounding factors.

Previous neuroimaging research has found a link between an individual’s intellectual capacity and the thickness of their brain’s cerebral cortex. For the present study, researchers scanned the thickness of several important regions of each subject’s brain, and then looked to see whether twins who abstained from drugs had thicker cerebral cortices than twins who drank or got high in their adolescence. 

The researchers found that twins who drank more alcohol as teens had less-developed cerebral cortices than twins who drank less, but found no link between pot use and cortical thickness. “No significant associations between cannabis use and thickness were observed,” the study authors wrote, according to NORML. “The lack of cannabis-specific effects is consistent with literature reviews, large sample studies, and evidence that observed cannabis effects may be accounted for by comorbid alcohol.”

These findings confirm the results of several other recent research studies. In 2017, a similar study found strong evidence that alcohol can damage the brain, but found no link between cannabis use and brain damage in adults or adolescents. And another twin study from 2016 also found zero evidence linking teen pot use to neurocognitive problems in adults.

In 2012, a team of Duke University researchers released a study suggesting that people who used cannabis as teens had lower IQs than those who did not. Prohibitionists embraced these findings, but the scientific community pointed out some serious flaws in this research. In 2019, the original study authors completely debunked their previous study with new research showing that these IQ deficits were caused by socioeconomic factors, not weed.

In addition to debunking myths about teen pot use, the present study provides unique insight into the dangers of underage alcohol abuse. “This study provides novel evidence that alcohol-related reductions in cortical thickness of control/salience brain networks likely represent the effects of alcohol exposure and premorbid characteristics of the genetic predisposition to misuse alcohol,” the study authors wrote. “The dual effects of these two alcohol-related causal influences have important and complementary implications regarding public health and prevention efforts to curb youth drinking.”