Cannabis is the most widely used drug in the world, with nearly 200 million people annually estimated to take it in some capacity — a figure that continues to grow as wider mainstream acceptance has given rise to pushes for legalized recreational use. But its popularity may come with significant cognitive consequences, as a new study found that heavy marijuana use during adolescence can lead to a decrease in IQ.

The research, published Jan. 27 in Psychological Medicine, studied more than 6,000 individuals across seven different cohort studies, and found that there was a correlation between frequent or dependent cannabis usage during youth and a decline in intellectual quotient of approximately two points on average.

Participants had their baseline IQ established prior to when they began engaging in frequent cannabis use, which was defined as using cannabis at least once a week for six months, and/or having greater than 25 reported lifetime uses, and/or receiving a formal diagnosis of cannabis dependency. Members of the control group reported having used little to no cannabis, with less than five uses reported in their lifetime. The participants’ IQ was then assessed again at a later follow-up date, which varied depending on the cohort study used; the age at follow-up ranged from about 18 years old to 38 years old.

The findings highlight the risks that heavy cannabis usage can present during youth, which are made potentially more dangerous due to how the brain is still developing at that time, according to Emmet Power, a clinical research fellow at the Royal College of Surgeons in Ireland and an author of the study.

A key issue in this line of research is that neurodevelopment doesn’t stop when a person turns 18 years old and is legally considered an adult, but continues until at least a person’s mid-20s, Power said. Early onset cannabis use has other associated risks, including other substance use disorders, major depression and poor educational or occupational outcomes.

“Emerging neuroimaging evidence also suggests that cannabis is associated with alterations in typical neurodevelopmental processes,” Power added. “There is significant biological plausibility in characterizing early and mid-adolescence as particularly high-risk periods in terms of cannabis use.”

Though cannabis has seen increasing use in a medical context, Power stressed that the potential risks for harm may outweigh any potential benefit of its usage, adding that quality data regarding medical marijuana is hard to come by.

“It’s interesting that so many jurisdictions have diverged from standard therapeutic drug approval practice around cannabis,” Power said. “This is not in line with my understanding of what good evidence-based medical research and practice is. The ethical principle of ‘First do no harm’ needs to apply not only to our patient interactions but also to our policy advocacy.”

Given that the study’s findings show how high levels of cannabis use can impact a developing brain, Power noted that there need to be better policies in place in order to protect adolescents, especially as the mainstream popularity of the drug continues to rise.

“The benefits and harms of legalization need to be much more clearly understood in differing contexts, and whenever I communicate on this topic I try and impart evidence about what I know about — specifically, the risks around cannabis use in young people,” Power said. “Ideally, any change in policy around substance use would be heavily weighted towards protecting, particularly, young people from harm and potential lifelong disability from severe substance use disorder or psychotic disorders related to substance use.”

The takeaway from this study is simply that frequent cannabis usage is associated with a decline in IQ, Powers said, but more work is yet to be done in researching the subject. The research team, which is based out of Ireland, is engaging with a coalition of youth, government, community, and public health stakeholders in order to implement a prevention program called Planet Youth, which is aimed at reducing youth drug usage.

“It’s something I am really excited about, as it has potential to significantly enrich young people’s lives in Ireland by focusing on protective factors through strong youth-community engagement and an interdisciplinary, capacity-building, evidence-based approach,” Powers said. “We are also looking at a number of different aspects of the effects of cannabis use on inflammation in youth and mental health associations of cannabis use in large longitudinal population samples of young people.”

The article, “Intelligence quotient decline following frequent or dependent cannabis use in youth: a systematic review and meta-analysis of longitudinal studies,” was published on Jan. 27, 2021 in Psychological Medicine. It was authored by Emmet Power, Sophie Sabherwal, Colm Healy, Aisling O’Neill, David Cotter, and Mary Cannon, all of the Royal College of Surgeons in Ireland.