Most potheads, cannabis connoisseurs, and patients, were most likely already confronted with the argument that cannabis can lower the IQ, often followed by the conclusion that it would be better to avoid smoking cannabis, to prevent brain damage and the decrease of intellectual abilities. This is probably one of the biggest myths about cannabis, and was told to many kids who grew up in the 80’s and 90’s.

Statements like this might be powerful and persuasive because basically no human being wants to be associated with a lack of mental sharpness but research is having a hard time to find evidence for this. It might be the case that parents, teachers, politicians, and all the other self-titled “cannabis-experts” using this argument, didn’t check the facts. We can be thankful for two dedicated teams of researchers from the United Kingdom, and from the US, for clearing up the smoke around the issue of declining IQs caused by cannabis consumption.

The last major scientific backup for this deep-rooted concept in society was a Duke University paper, published in the Proceeding of the National Academy of Sciences, in 2012. It concluded that frequent and heavy marijuana use is likely associated with a decline in IQ. The same journal later published a follow-up study, saying that many factors that also have been linked to IQ decline, such as cigarette and alcohol use, low socio-economic status, and mental illness, were not taken into consideration.

This doesn’t necessarily makes the Duke University study fundamentally flawed but Ole Rogeber, lead author of the follow-up study, gives hints that there were some problems with it: “Although it would be too strong to say that the results have been discredited, the methodology is flawed and the causal interference drawn from the results premature,”

There are two new scientific reports that examine the link between cannabis use and intelligence from slightly different angles: One looks at the development of 2,235 teenagers from the UK between ages 8 and 16, and the other one examines the differences between 789 identical twin pairs in the US, in which one twin smokes cannabis, and the other one does not.

Comparing two identical twins can be a good tool to examine the impact of cannabis on intelligence because the genetic makeup of twins is close to being identical, and their early social environment is consistent.

Both of these studies came to the same result: There is no evidence that cannabis use among teenagers leads to a decline in intelligence.

teen cannabis IQ

Researchers of the British report published their results in the Journal of Psychopharmacology: “cannabis use by the age of 15 did not predict either lower teenage IQ scores or poorer educational performance. These findings therefore suggest that cannabis use at the modest levels used by this sample of teenagers is not by itself causally related to cognitive impairment.”

Authors of this study state that their data and analyses don’t necessarily invalidate findings of the scientific paper released by Duke University. The British report only examined moderate use of cannabis whereas scientists from Duke University focused on heavy use, over a long period of time. “While persistent cannabis dependence may be linked to declining IQ across a person’s lifetime,” the authors write, “teenage cannabis use alone does not appear to predict worse IQ outcomes in adolescents.”

Results of the longest ever longitudinal twin study, by US scientists from California and Minnesota, are more in contrast to what scientists from Duke University concluded in their paper. The twin data doesn’t support the implication by authors of the Duke study that cannabis use in adolescence causes neurocognitive decline. IQ scores of twins who smoked weed didn’t decline any more over time, than the ones from their non-using siblings. Both smokers and non-smokers lost about 4 IQ points. Scientists found something different: “children who are predisposed to intellectual stagnation in middle school are on a trajectory for future marijuana use.”

In other words, kids who are having a hard time in school, for whatever reason, are more likely to try smoking weed at some point of their life. If cannabis use is causing a cognitive decline, one would expect heavy users to have a greater decline in intelligence, than moderate or non-frequent users. Analyses of the twin data couldn’t find evidence for this either. Researchers found that heavier cannabis use was not associated with greater decreases in IQ. There must be something else that led to the decline of 4 IQ points in both groups. Statistician and lead author Nicholas Jackson concludes: "Our findings lead us to believe that this ‘something else’ is related to something about the shared environment of the twins, which would include home, school, and peers,"

Cannabis science is making progress but is still not at a point where it needs to be. This twin study from America, or the study by British researchers, are maybe more precise from a scientific point of view but they still have their limitations. The one thing is: Do IQ scores accurately reflect the intelligence of a human being? There are many scientists that don’t believe in this concept and refer to this common assumption as “the IQ myth”. The other thing is that findings of these studies can’t provide proof that heavy and persistent cannabis use throughout adolescence is safe.

There are multiple risks associated with cannabis but for now, we can take a deep breath, and be happy about the fact that our stupidity is most likely caused by something else, and not by the plants we like to grow and smoke. "We desperately need more research on the effects that marijuana has on the brain," says Jackson, lead author of the twin study.


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