Taking a toke or two from a measly joint may not seem like a big deal compared to the various ways you can use the drug. But to teenagers, whose bodies are still developing, even a tiny bit of cannabis use appears to have profound effects. As a new Journal of Neuroscience study on the link between cannabis and teen brain volume shows, a little bit of weed can go a long way.

Brain volume and cannabis use have been linked in previous studies, but the authors of the new paper were particularly interested in how much — more specifically, how little — it actually takes for the drug to exert its effects. The short answer is: Not much. The senior author of the paper, University of Vermont psychiatry professor Hugh Garavan, Ph.D., tells Inverse, “we were surprised by the large effects we found.”

"It was surprising to think that these could be the result of just one or two uses.

Previous studies on the link between brain volume and teen marijuana use is “quite varied,” says Garavan, but that’s partially because of poor study design that didn’t control for other confounding variables, not because the relationship doesn’t exist. To figure out whether “extremely low levels of cannabis” affected the brain volume of the teens in the study, the authors, led by Catherine Orr, Ph.D., had to control for confounding factors like age, sex, socioeconomic status, IQ, and alcohol and nicotine use.

Even a history of smoking just one or two joints was associated with changes in grey matter volume. Unsplash / Wesley Gibbs

Bigger Isn’t Better

And so, Garavan and the study team took advantage of a huge longitudinal data set of brain scans of European teens known as IMAGEN, choosing 47 kids who had used cannabis once or twice by the time they were 14. Their analysis confirmed their hypothesis: “We see very many brain regions in which grey matter volume is greater in the cannabis users and it was surprising to think that these could be the result of just one or two uses (joints),” says Garavan. Grey matter refers to the darker part of the brain, which is where all its synapses live.

It may seem counterintuitive that cannabis use is associated with an increase in brain volume. But during a person’s teenage years, their brains are actually getting smaller in size. “At the age at which we studied these kids (age 14) cortical regions are going through a process of thinning – the idea is that this is a “sculpting” process that makes the brain and its connections more efficient,” says Garavan.

“If cannabis is affecting this process then it is reasonable to suspect that it could lead to cognitive differences.”

While we don't know *how* brain volume is affected by cannabis, Garavan recommends young teens lay off the drug. University of Vermont Larner College of Medicine

Too Early For Cause and Effect

That said, Garavan is careful to point out that the team’s small study shows only an association between increased brain volume in specific areas and low-level cannabis use, not a causal relationship in which cannabis brings about the change. They noticed changes in brain volume in the amygdala, which is involved in processing fear and emotion, and the hippocampus, which deals with memory and spatial ability; the striatum, bilateral parietal regions, and parts of the cerebellum and left middle temporal gyrus were also affected.

“We don’t know the exact mechanisms that underlie the increased volume nor indeed do we know for sure that the increases are a consequence of the cannabis use (but that is what our data suggest),” says Garavan.

"It would seem wise to encourage young teens to avoid cannabis.

Writing about what it means to have increased grey matter volume in the areas they studied, the authors do note a general relationship with lower IQ and psychomotor skills. However, Garavan points out, “to be clear, we do not see deficits on these measures in the present group of cannabis users. The implication is that if we had more participants (i.e., better statistical power) then we might detect group differences.”

The study raises more questions than it answers, but it highlights a research question that gets more important with every passing legalization bill. “Given that we don’t understand the exact brain mechanisms underlying the observed effect and we don’t know what explains the differences between users, we should be cautious before drawing firm conclusions,” he says.

“That said, the results suggest that very light cannabis use may have an impact on brain structure and perhaps especially so in young users (age 14). It would seem wise to encourage young teens to avoid cannabis.”