Cannabis does not affect the intelligence of teenagers


One of the big myths about cannabis and which has become a pop culture cliché is the assertion that its consumption has a negative impact on intelligence, especially during adolescence. Last January, the results of two studies on teenagers were published which found that the consumption of cannabis cannot be blamed for a lower IQ or poor performance in school.

Since the ban, the propaganda against this plant, emboldened by the “Reefer Madness” era has described the plant as a substance which undoubtedly caused mental harm, especially to teenagers and young people. Over the decades, this idea became widespread, perpetuating this myth and turning it into a pop culture cliché. This has resulted in the lingering and widespread worry amongst teenagers that by consuming cannabis, they are exposing themselves to various cognitive and psychological effects.

Cannabis Does Not Affect the Intelligence of Teenagers
Anti-cannabis propaganda during the Reefer Madness era alleged that cannabis caused mental damage to young people (CC. Chuck Cocker)

Although studies have been published on the cognitive effects of the plant which associate it with cognitive deficits, such as memory loss and a low intelligence quotient, many of these studies are methodologically inconsistent, and show contradictory results. Thus, it remains controversial whether there is actually any real risk.

Confirmation from Two Scientific Studies

In January this year, the results of a study were published which once again examined the consumption of the plant during adolescence. Finally, a group of scientists has gone further and conducted the first study of its type, which analysed the long-term consumption of cannabis in twin teenage siblings. The study compared the changes in IQ in one of the twins who consumed cannabis against the other twin, who did not consume cannabis for a period of 10 years.

The results were presented a few days after the publication of a similar study in which a group of investigators from the United Kingdom came to the conclusion that the consumption of cannabis has no effect on the IQ or the school performance of teenagers between the ages of 15 and 16.

The conclusions reached by both groups of scientists refute the findings of previous studies, which asserted that the consumption of cannabis may damage the development of teenagers’ brains. The two studies conclude that the consumption of the plant is not a contributing factor towards a reduction in the intelligence quotient (IQ) nor a deterioration in school performance.

A scientific study on teenage twins

January 2016 saw the publication of a study called: “Impact of adolescent marijuana use on intelligence: Results from two longitudinal twin studies” in the Proceedings of the National Academy of Sciences. The research group led by Nicholas J. Jackson from the University of Southern California (USC), conducted a longitudinal study to evaluate the potential effects of cannabis consumption in the intelligence quotient of teenage identical twins, who share genes and educational background; one twin consumed cannabis on a regular basis whereas the other never consumed it.

To conduct this new study, the investigators compiled statistics from the other two studies on teenage twins in order to analyse them. They focused on slightly more than 3,000 people – 3,066 adolescent subjects to be precise – who underwent tests on intelligence when they were between the ages of 9 and 12, obviously before they started consuming the plant. Over 10 years, the research group carried out five intelligence tests and confidential surveys on the use of cannabis. The subjects were also asked about their use of other substances, such as painkillers, opiates, cocaine and excessive alcohol consumption.

Subsequently, the results of these tests were compared with the tests which were carried out when the participants were between 17 and 20 years of age. Changes in the subjects’ IQ test scores were recorded to determine the difference in the development of twins who consumed cannabis with those who abstained from it, and whether development was impaired in the case of the consumers.

Cannabis Does Not Affect the Intelligence of Teenagers
In the span of 10 years, various intelligence tests and surveys on the consumption of cannabis and other substances have been conducted (CC. Alberto G.)

According to the investigators, when they analysed the effects of cannabis consumption in the twins they were able to adjust for potential disrupting variables, such as aspects including family background and genetic propensity. Given that twins are normally brought up in the same household and share most, if not all, of the same DNA components, the investigators were able to carry out a more precise analysis during their comparison.

They found that subjects who started consuming cannabis during the target period had a tendency to have lower levels of intelligence than normal. However, they discovered that twins who did not consume cannabis also had low levels of intelligence, which suggested that the low level of IQ development was not the result of cannabis consumption. Rather, this factor could have been affected by a range of contextual factors, such as the family environment or genetic predisposition. Thus, consumption cannot be considered as a main factor for the low intelligence levels.

What Jackson and his team found was that there was no dose-response relationship between cannabis consumption and the deterioration in the intelligence quotient. Moreover, they also confirmed that there was no significant difference in intellectual performance between subjects who consumed cannabis and their twins who did not. This correlation is even applicable to those who smoked more than 30 times or those who consumed cannabis daily for over six months.

Finally, the authors of the study emphasised that their research differed from other previous studies on the same subject, as theirs was a longitudinal study rather than a cross-sectional one. What this means is that, this study followed the behaviour and development of subjects over a long period of time, rather than merely observing a group of participants at a given moment. This focus provides the investigators with a more reliable set of data, which enables them to observe how a range of factors can impact the development of teenagers and allows them to identify the real causes of IQ reduction, rather than blaming cannabis for it.

Although the consumers of cannabis lost around four IQ points during the course of the study, their abstaining twins presented a similar deterioration, which suggests that the loss of mental acuity was linked to something other than cannabis, says Jackson. “Our results lead us to believe that this ‘something else’ is linked to the shared environment of the twins, which includes the home, school and friends.”

Cannabis Does Not Affect the Intelligence of Teenagers
Two studies found that cannabis was not a causal factor in lower IQ levels (CC. David Mulder)

Another scientific study on teenagers

Just a few days before, also in January 2016, the results of a similar study, were published under the title “Are IQ and educational outcomes related to their cannabis use? A prospective cohort study” in the magazine Journal of Psychopharmacology, which basically presented the same conclusions as Jackson’s team.

The study, conducted by British scientists, tried to address the ongoing debate regarding the impact of cannabis consumption in the intellectual and academic performance of teenagers. The team investigated the relationship between cannabis consumption in teenagers between 15 and 16 years of age, and their intelligence quotient and school performance. A group of 2,235 British teenagers, who are not twins, took part in the study. Of this group, 24% said they had tried cannabis at least once.

Through a statistical analysis and after adjusting for potential confounding factors, such as childhood depression, behavioural problems, and the consumption of cigarettes and alcohol, the investigators found that the results of teenagers who had consumed cannabis 50 times or more did not differ from the results of those who had never consumed it, both in terms of IQ and school performance.

Interestingly, even after the sample of results from cannabis consumers had been excluded, tobacco smokers fared significantly worse in terms of school performance, which shows that there is a solid link between cigarette consumption and school performance. All this suggests that cannabis consumption is not a predictor of low IQ levels in teenagers; there is no link with a reduced IQ nor does it result in a worse school performance for teenagers.

Cannabis Does Not Affect the Intelligence of Teenagers
Teenage tobacco smokers performed worse in school than cannabis consumers (CC. Brainbitch)

The investigators concluded: “In summary, the notion that cannabis use itself is causally related to lower IQ and poorer educational performance was not supported in this large teenage sample.”

The significance of these findings

There are numerous clichés and erroneous ideas that the defenders of prohibition have deployed and which have gained traction amongst the general public. However, it is becoming increasingly harder for them to stand by these allegations, which thanks to current research, have been refuted. These scientific studies are crucial for clearing up doubts and assuaging the fears of people who are reluctant to support the removal of the existing strict regulations on cannabis world-wide.


Although the movement in favour of the legalisation continues to grow and work towards a better knowledge and understanding of the effects of cannabis, and the medical research community continues to make innovative discoveries, what is needed is further research on effects of cannabis on cognitive capacities. However, this would require it to be administered to subjects in order to see how the duration, frequency and dosage really affect the brain. Unfortunately, this type of study is practically impossible to carry out due to current restrictions. Obviously, we need a clear legislative framework to regulate cannabis for a host of reasons.


The post Cannabis does not affect the intelligence of teenagers appeared first on Sensi Seeds Blog.

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