While the recreational use of marijuana is an increasingly legalized activity, the exact science of what the drug does to the brain isn’t yet conclusive. In an effort to understand what happens to the brain right from the start of smoking, scientists recently studied the substance use of nearly 4,000 teenagers in Canada. Their research indicates that marijuana not only affects teenagers’ cognitive abilities, it also has more long-term consequences on working memory than alcohol.
In the study, published Wednesday in the American Journal of Psychiatry, scientists from CHU Sainte-Justine and the University of Montreal determined that when teenagers consistently and increasingly use marijuana, cognitive functions like recall memory, perceptual reasoning, inhibition, and working memory are damaged. While both alcohol and marijuana affect how teen’s brains work, senior author Patricia Conrod, Ph.D. said Wednesday that “increases in cannabis use, but not alcohol consumption, showed concurrent and lagged effects on cognitive functions.”
For the study, Conrod and her team evaluated 3,826 seventh grade students from 31 schools in the Greater Montreal region for four years. Once a year, the researchers would ask the students to rate the frequency of their marijuana and alcohol consumption on a six-point scale. Students were assured that parents and teachers wouldn’t have access to the information unless their habits indicated an imminent risk of harm.
Next, the teens’ cognitive functions were assessed through a variety of memory tests. For example, a delayed recall test involved reproducing a previously learned pattern of stimuli 30 minutes later. In a perceptual reasoning test, teens were asked to complete a sequence of increasingly difficult puzzles, and in an inhibitory control test, teens had to learn by trial and error when to respond to “good” or “bad” numbers.
These tests, paired with the reports about the teens’ substance use, revealed that individuals who used cannabis and alcohol were more likely to show worse working memory, perceptual reasoning, and inhibitory control. The findings are in line with previous research, but the novel part of the study is the revelation that cannabis could be worse for growing brains.
“Over and above the effect of being prone to cannabis use during adolescence, when increases in cannabis use frequency were observed in a given year, reductions in delayed recall memory and perceptual reasoning were observed in that same year, and these effects were independent of any changes in alcohol quantity and frequency,” the authors write.
This conclusion differs from previous research that touches on a much-debated question: What’s worse for your brain, marijuana or alcohol? A 2017 study from the University of Colorado-Boulder came to the opposite conclusion, suggesting that the consumption of booze is more detrimental to brains than cannabis. When these scientists looked at the brains of 850 adults and 430 teenagers they found that alcohol led to lower volumes of grey and white matter — essential tissues that affect brain function. Smoking marijuana didn’t result in the same losses.
However, the National Institute on Drug Abuse maintains its stance that “substantial evidence from animal research and a growing number of studies in humans indicate that marijuana exposure during development can cause long-term or possibly permanent adverse changes in the brain.” More studies are needed — especially those that will continue to follow teenagers as they encounter easier access to substances in college — in order to really know the effects these drugs have on cognitive functions. Until then, it can be said that smoking marijuana does come with cognitive repercussions.