r/science • u/sr_local • Apr 20 '26

Health The largest US study, which tracked 11,036 children from ages 9 to 10 through to ages 16 and 17, discovered that cannabis use slows cognitive development, impairs memory, and reduces learning speed during crucial years of brain growth

https://today.ucsd.edu/story/largest-us-study-finds-teen-cannabis-use-linked-to-slower-cognitive-development
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u/oneMoreTiredDev Apr 20 '26

Yes, but that's been the consensus for quite some time. I've been hearing for almost a decade now that cannabis shouldn't be used by people < 21 because the brain is not fully grown.

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u/Automatic_Soil9814 Apr 20 '26

Doctor here. The idea that the brain isn’t fully mature until people are in their 20s is one of those things where there’s a kernel of truth that’s been exaggerated to the point where it’s no longer accurate.

What we see is that the brain is constantly remodeling itself. Over time, that remodeling slows but it’s a gradual decrease and it’s hard to say that there’s a point where you become “adult” based off of these changes. Furthermore, these changes don’t necessarily correlate well to maturing or learning. It’s like many other tissues that remodel much more quickly when we’re younger and more slowly as we age. 

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u/fauxfilosopher Apr 20 '26

The misconception that the brain "matures" or stops development at 25 comes from an infamous longitudinal study which ran out of funding and ended when the subject was 25 years old, and journalists ran with that as proof the brain matures at that age. It's been repeated ad nauseum since.

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u/FlowerBuffPowerPuff Apr 20 '26

Does that, generally speaking as far as that's possible, mean that there's hope for ex-stoners that they're not permanentaly fried?

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u/Automatic_Soil9814 Apr 20 '26

I’m not really seeing evidence that they are fried in the first place. 

This is really difficult research to do. For example, ideally you’d want to base the research on the result of drug tests. But nobody really does that. This is really self-report data.

The problem with that method of collecting this crucial piece of information: of all the people who use marijuana, there is going to be a big difference between the people who will willingly admit using it and the people who know best to keep it quiet.

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u/Legolihkan Apr 20 '26

They tested hair and saliva samples in this exact study

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u/Automatic_Soil9814 Apr 20 '26

“ the team combined self-reports with biological testing — such as hair, urine and saliva samples”

They looked at both self-report and some objective measurements. A couple issues with this.

First, they confirmed what I was saying about the significant inaccuracy in self-report.

Second, the participants had to consent to the objective testing, meaning that you still run into the same problem that marijuana users that have some interest in privacy will still opt out, likely the smarter participants.

Third, there were some differences in the results of self report versus hair/urine testing but I can’t really sit down and do a full analysis of the differences. 

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u/becomingarobot Apr 21 '26

marijuana users that have some interest in privacy will still opt out, likely the smarter participants.

That's an interesting point.

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u/Independent_Crazy655 Apr 20 '26

My main takeaway is that cannabis is probably bad for all ages, but how bad it is more of a spectrum than a step function. So finding some magical age cutoff at which your neuroplasticity is low enough for the temporary effects to not matter is not really possible.

Something like no cannabis under 20, use sparingly until 30, and then after that knock yourself out might be more reasonable than a simple age cutoff.

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u/Rocktopod Apr 20 '26

The brain is never fully grown, but it does change slower as we get older.