New Study Debunks Myth: Smoking Weed Does Not Make You Stupid

Long-term cannabis use does not cause cognitive decline. This is the finding of a new study conducted on twins.

The study looked at sets of twins where only one sibling uses cannabis and the other doesn’t. This way the researchers were able to control for genetic differences and childhood environment. 

While previous research has controlled for factors such as education levels, mental health and concurrent drug use, this study explores whether there could be a connection between a genetic predisposition for cognitive decline and likelihood of using cannabis.

Previous research 

A number of studies have found a link between regular cannabis use and a decline in cognitive function and IQ. The link is particularly strong among adolescent consumers. 

However, these studies have been unable to confirm that cannabis is the cause of the cognitive decline. So, while there is a clear association between adolescent cannabis use and a decline in cognitive function, a causative relationship has not been established. 

One such study from 2012 found small but significant decreases in cognitive function in subjects who started toking at a young age. The more they consumed, the larger the decline in cognitive function, the study noted. 

Studies like these may not be as clear-cut as the seem though. That’s because, due to experimental limitations, they can’t prove a causal relationship. And as evidence from the new study suggests – as well as from a number of previous twin studies – that’s because there isn’t one. 

Similarly, a strong link has been found between psychosis and cannabis use, yet a causal relationship is proving harder to establish – possibly because it doesn’t exist. 

Additionally, there is some evidence that occasional cannabis consumption may even improve cognitive function in adults.

New twin study

This recent study is important because, by looking at hundreds of pairs of twins, researchers were able to largely control for vital variables that could muddy the results. Variables like childhood environment and genetics. 

They also focused on sets of twins where only one sibling was a cannabis consumer. By comparing such pairs of twins, researchers were able to fairly compare cognitive discrepancies between then siblings. 

If cannabis use does indeed cause cognitive decline, it should be pretty apparent that the sibling who consumes cannabis has decreased cognitive function compared the their sibling.  

Similarly, the cannabis-abstaining twin should have a higher score on cognitive measures than the other one.

That’s not what happened though. Rather, the siblings did not differ much in their cognitive abilities despite one consuming cannabis.

Researchers concluded, “We found little support for a potential causal effect of cannabis use on cognition, consistent with previous twin studies. Results suggest that cannabis use may not cause decline in cognitive ability among a normative sample of cannabis users.”


Although causation was not established, the researchers did find a reduction in adulthood IQ and executive functioning that was correlated with cannabis use in youth. However, in pairs of twins where this was seen, both twins experienced the cognitive decline. 

The researchers suggest that other factors like genetics or environment predispose some people to both cannabis use and a decline in IQ in adulthood. 

One exception was found though. In an isolated result, the researchers report that cannabis use at age 17 was correlated with reduced cognitive ability at age 23. 

As the researchers note, this study was limited by its relatively small size and limited sample pool (predominantly white), so this result could be down to not having a diverse enough sample pool.

One other issue with the study may the fact that not many of the subject were daily cannabis consumers – although many were near-daily users. More research may be needed to see if these results are consistent with heavy cannabis consumption. 

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