Persistent cannabis users show neuropsychological decline from childhood to midlife

https://www.pnas.org/con...ly/2012/08/22/1206820109

Abstract

Recent reports show that fewer adolescents believe that regular cannabis use is harmful to health. Concomitantly, adolescents are initiating cannabis use at younger ages, and more adolescents are using cannabis on a daily basis. The purpose of the present study was to test the association between persistent cannabis use and neuropsychological decline and determine whether decline is concentrated among adolescent-onset cannabis users.

Participants were members of the Dunedin Study, a prospective study of a birth cohort of 1,037 individuals followed from birth (1972/1973) to age 38 y. Cannabis use was ascertained in interviews at ages 18, 21, 26, 32, and 38 y. Neuropsychological testing was conducted at age 13 y, before initiation of cannabis use, and again at age 38 y, after a pattern of persistent cannabis use had developed. Persistent cannabis use was associated with neuropsychological decline broadly across domains of functioning, even after controlling for years of education.

Informants also reported noticing more cognitive problems for persistent cannabis users. Impairment was concentrated among adolescent-onset cannabis users, with more persistent use associated with greater decline. Further, cessation of cannabis use did not fully restore neuropsychological functioning among adolescent-onset cannabis users.

Findings are suggestive of a neurotoxic effect of cannabis on the adolescent brain and highlight the importance of prevention and policy efforts targeting adolescents.

I'm not completely surprised. Cannabis does make you lazy and less focussed. That could be caused by neurological decline, but it could also be the main cause of it itself.

Overall the study seems well performed, though some critical analysis leaves something to be desired re: the researchers' conclusion of neurotoxicity.

Funding from NIMH and NIDA. Surprise!

IQ test scores between childhood and adulthood as the main measure of cognitive impairment. Self-report and third party report inventories making up the remaining measures.

No controls for lifestyle, education, nutrition, or many of other potentially confounding variables, only minimal controls for alcohol and other drug use.

No actual analysis for neurotoxic effects of cannabis whatsoever.


Did the researchers measure what they set out to measure (cognitive impairment)?
Arguably, Yes.

Does the evidence appear to support the hypothesis?
Yes, the data appears to support that more consistent cannabis use is correlated with lower scores on on the selected measurement instruments.

Does the evidence reflect the researchers' conclusions?
No, these measures have little if anything to do with neurotoxicity.

Just goes to show how important it is use critical thinking skills when evaluating research publications

P.S. - Don't give weed to children, it's probably not a good look.

thanks for the analysis, dreamoar! have to say it was what i expected when i skimmed the beginning and got distracted by my vape. so thanks for critically reviewing it.

all the same personally, I wouldn't recommend marijuana to anyone under 25, which is when the brain stops developing. but who knows. it is certainly not out of the question that there may be conditions and circumstances in which it could be beneficial for a still growing brain but we're not at the point of having such refined knowledge yet.(i started at 20, i had no idea back then... oh well)

dragonrider, i would respectfully disagree that cannabis "makes anyone" anything... it can be used in so many different ways - including enhancing laziness, yes, but that is but one of many...

I also don't believe that there's any indication for neurotoxicity there.

We all know cannabis is a mild anticholinergic, so it is far more likely that this is an accumulated effect of a lack of proper "mental exercise": consistently performing and functioning below your actual level...say at school, work, uni, and anywhere else where you may be challenged, intellectually.

I think statistically, more people will find it sedating than stimulating. So generally speaking, it will make people lazy and unfocused. Maybe not everybody, all the time. But most people, most of the time.

The cliche image, people have of a stoner (and people have this image, not exactly out of ignorance, because weed is everywhere), is not that of a nuclear physicist or an olympic athlete.

It can be used as a stimulant. But most of the time it definately isn't.

Neurotoxic is a word I suspect may lack aptitude in this.
However I have encountered a lot of people who appear to have some form of issues with cognitive development that have smoked cannabis from an early age... yet nearly every one of them also used alcohol from an early age and that is known and shown to have a plethora of negative effects. It is not kind to its users. I suspect alcohol may be a major factor here.
I have not heard of a fetus being seriously harmed in its development by cannabis, however alcohol has a pronounced negative impact upon fetal development to the point that I have known women to use it to cause spontaneous abortion. Those negative effects upon development still exist in children and teenagers and the health issues associated with it remain serious for adults. The recent World Health Organization study comes to mind.

Lowered IQ is well demonstrated for alcohol. If cannabis is going to be implicated for harming people then I want to be able to rule out alcohol use.

Yet another flawed "study" that knew what they wanted to find and surprise, that is exactly what they found. There is little to no science or medicine in paid junk studies such as this and they should all be discredited as the pseudo-science that they are.

I agree with what you guys said that you gotta be careful with conclusions and read the whole thing critically.

That being said, just for the sake of accuracy, dreamer042, they did check for education effect on it (bottom of page 4), and Mitakuye Oyasin, they did also consider the effect of alcohol use (dependence) and the negative cannabis effects were there.

The study had a quite large sample, checked for other relevant confounding variables, and followed people before and after use for many years, all of which are positive aspects of this study. I'd like to see more research on this to see if this can be found by independent researchers considering other confounding factors.

Either way, I think it's prudent to try to delay initial use of cannabis with teenagers, and I will certainly try to convince my son when he's that age, to wait as much as possible if he does want to try weed.

Informative reply.
I have encouraged my children to wait until they are around 18 to experiment with Cannabis.
I do believe it has negative effects upon development and cognition.
I was a daily user for 15+ years and often used many grams a day.

I find the term Alcohol Dependence to be vague, negative effects upon development can occur without dependence. In fact some of the negative effects of alcohol are most pronounced in users with low tolerance, ergo no dependency. And extremely negative health effects are known for alcohol use when used intermittently in binge drinking which is rather common among young drinkers. That being said I have met numerous pot heads that issues with thinking, and a few, a clear minority who were brilliant exceptions.

THC was recently tested directly on adolescent rats. They did find negative cognitive effect which seemed to diminish over time to some extent.

In short, there is mounting evidence that adolescents should not take marijuana without weighting this possible side effect. For example, if marijuana can help with severe seizures, the benefits may outweigh the potential side effects.

I think decriminalization would allow to regulate access and also focus on the science we have. Criminalization confuses reality with propaganda (in my oppinion) and creates an underground market that makes it very easy for underage people to access these substances (iny experience). In Canada, these presumptive effects were used as arguments for the recent marijuana legalization.

In short, decriminalize and focus on the science and real fact based education on drugs seems like a good path we should be moving towards (I think).

Once the observations (to the best of our understanding) are layed out, the adult individual should have the freedom to review them and make their own personal decision without fear of criminal prosecution. I believe this to be a self-evident principle.