DUBLIN, Ireland — While nations argue over how intelligent it is to legalize marijuana, a new study finds the drug may literally be impacting the intelligence of young users. Researchers from the Royal College of Surgeons in Ireland (RCSI) say frequent use of cannabis during adolescence can lead to having a lower IQ as an adult.

Researchers have discovered that regular pot smokers suffer a decline of two IQ points over time compared to those who did not use cannabis as an adolescent. Further analysis revealed verbal IQ, which refers to understanding concepts, abstract reasoning, and working memory, declines by three points.

While that may not sound significant, study authors say even minimal incidental contact with marijuana can result in morphological brain volume changes. An analysis of MRI studies discovered those who smoked weed showed evidence of reduced grey matter in the brain.

“Previous research tells us that young people who use cannabis frequently have worse outcomes in life than their peers and are at increased risk for serious mental illnesses like schizophrenia,” says senior author Professor Mary Cannon in a university release. “Loss of IQ points early in life could have significant effects on performance in school and college and later employment prospects.”

Is cannabis a growing problem among America’s youth?

According to the National Institute on Drug Abuse, 11.8 million young adults in the U.S. reported using marijuana in 2018. It is the most commonly used psychotropic drug in the country, behind only alcohol.

The RCSI team studied 808 young people who used cannabis at least weekly for a minimum of six months. They also examined 5,308 young people who did not use the drug.

The results reveal significant differences in terms of IQ between the two groups. Researchers say, among frequent or dependent cannabis users, overall IQ dropped by 1.98 points. When examining the impact on verbal IQ using four available studies, the team discovered a 2.94-point drop in these scores.

“Cannabis use during youth is of great concern as the developing brain may be particularly susceptible to harm during this period. The findings of this study help us to further understand this important public health issue,” explains study first author Dr. Emmet Power.

‘Approximately two-point decline in IQ’ among teens who frequently use marijuana

The researchers note many of the studies they examined provide limited data on this issue. Only one followed adolescents beyond the age of 18; with that one tracking their intelligence until age 38. With that said, researchers believe all the negative effects of frequent cannabis use early on are still unknown.

“The approximately two-point decline in IQ in adolescent-onset frequent cannabis users is not to be clinically significant and alone is unlikely to completely explain a range of psychosocial problems linked to cannabis use in this cohort,” the team writes in the journal Psychological Medicine.

“Developmental effects, however, such as altered neuromaturational processes may not be fully captured by periods of follow up limited to adolescence when brain development is ongoing.”

SWNS writer Joe Morgan contributed to this report.

Our Editorial Process

StudyFinds publishes digestible, agenda-free, transparent research summaries that are intended to inform the reader as well as stir civil, educated debate. We do not agree nor disagree with any of the studies we post, rather, we encourage our readers to debate the veracity of the findings themselves. All articles published on StudyFinds are vetted by our editors prior to publication and include links back to the source or corresponding journal article, if possible.

Our Editorial Team

Steve Fink


Chris Melore


Sophia Naughton

Associate Editor