Study shows no relationship between moderate adolescent cannabis use and exam results, IQ

October 20, 2014

A large UK study has found that occasional adolescent cannabis use does not lead to poorer educational and intellectual performance, but that heavy cannabis use is associated with slightly poorer exam results at age 16. The results come from the Avon Longitudinal Study of Parents and Children (ALSPAC, also known as "Children of the 90's") a long-term study that follows the health of children born in the Bristol area (UK) in 1991 and 1992. The work is being presented at the annual congress of the European College of Neuropsychopharmacology (ECNP) in Berlin.

The researchers analysed data from 2,612 children who had their IQ tested at the age of 8, and again at the age of 15. These children's examination results were then factored in via the National Pupil Database. At the age of 15, each person in the study completed a survey on use. The researchers then used regression analysis to look at how cannabis use affected both intellectual and . A number of children could not be included in the final analyses (for example because they had experienced a head injury), leaving a total sample size of 2,235.

The researchers found two main points

  • Cannabis use appeared to be associated with decreased intellectual performance. Cannabis use was, however, highly correlated with other risky behaviours such as alcohol, cigarette and other drug use. When the researchers took these other behaviours into account, they found there was no relationship between cannabis use and lower IQ at age 15.
  • Heavier cannabis users (at least 50 times by age 15) however, did show marginally impaired educational abilities. These tended to have poorer exam results (3% lower) on compulsory school exams taken at age 16, even after adjusting for childhood educational performance, as well as alcohol, cigarette and other drug use.

According to lead researcher, Claire Mokrysz (University College London):

"Our findings suggest cannabis may not have a detrimental effect on cognition, once we account for other related factors- particularly cigarette and alcohol use. This may suggest that previous research findings showing poorer cognitive performance in may have resulted from the lifestyle, behaviour and personal history typically associated with cannabis use, rather than cannabis use itself.

People often believe that using cannabis can be very damaging to intellectual ability in the long-term, but it is extremely difficult to separate the direct effects of cannabis from other potential explanations. Adolescent cannabis use often goes hand in hand with other drug use, such as alcohol and cigarette smoking, as well as other risky lifestyle choices. It's hard to know what causes what- do kids do badly at school because they are smoking weed, or do they smoke weed because they're doing badly? This study suggests it is not as simple as saying cannabis is the problem.

This is a potentially important public health message- the belief that cannabis is particularly harmful may detract focus from and awareness of other potentially harmful behaviours. However the finding that heavier cannabis use is linked to marginally worse educational performance is important to note, warranting further investigation".

Commenting ECNP Chair, Professor Guy Goodwin (Oxford) said

"This is a potentially important study because it suggests that the current focus on the alleged harms of cannabis may be obscuring the fact that its use is often correlated with that of other even more freely available drugs and possibly lifestyle factors. These may be as or more important than cannabis itself".

The researchers noted that the study has some limitations. For example, cannabis use was self-reported, and the measure of IQ taken at 15 was an abbreviated version of the standard Wechsler IQ test. Full details can be found in the abstract (however please note that the abstract shows a preliminary analysis; this may differ from the version which is currently being prepared for publication and which is described above).

Explore further: What 20 years of research on cannabis use has taught us: review

Related Stories

What 20 years of research on cannabis use has taught us: review

October 7, 2014
In the past 20 years recreational cannabis use has grown tremendously, becoming almost as common as tobacco use among adolescents and young adults, and so has the research evidence. A major new review in the scientific journal ...

Frequent cannabis use in adolescence linked with reduced educational attainment

September 10, 2014
Individuals who are daily users of cannabis before age 17 are over 60% less likely to complete high school or obtain a degree compared to those who have never used the drug, new research published in The Lancet Psychiatry ...

Aussie cannabis users found to self-medicate

September 3, 2014
One in two Australians who grow cannabis for medicinal reasons are doing so without speaking to their doctor, according to responses from cannabis growers to an international survey.

Smoking cannabis doesn't make you more creative

October 8, 2014
People often think that smoking cannabis makes them more creative. However, research by Leiden psychologists Lorenza Colzato and Mikael Kowal shows that the opposite is true. They published their findings on 7 October in ...

German study finds cannabis use triggered 2 deaths

February 26, 2014
(AP)—Scientists in Germany say cannabis use has been linked to the deaths of two outwardly healthy young men later found to have underlying conditions.

Schizophrenia and cannabis use may share common genes

June 24, 2014
Genes that increase the risk of developing schizophrenia may also increase the likelihood of using cannabis, according to a new study led by King's College London, published today in Molecular Psychiatry.

Recommended for you

Simple EKG can determine whether patient has depression or bipolar disorder

November 20, 2017
A groundbreaking Loyola Medicine study suggests that a simple 15-minute electrocardiogram could help a physician determine whether a patient has major depression or bipolar disorder.

Non-fearful social withdrawal linked positively to creativity

November 20, 2017
Everyone needs an occasional break from the social ramble, though spending too much time alone can be unhealthy and there is growing evidence that the psychosocial effects of too much solitude can last a lifetime.

Car, stroller, juice: Babies understand when words are related

November 20, 2017
The meaning behind infants' screeches, squeals and wails may frustrate and confound sleep-deprived new parents. But at an age when babies cannot yet speak to us in words, they are already avid students of language.

Cultural values can be a strong predictor of alcohol consumption

November 20, 2017
Countries with populations that value autonomy and harmony tend to have higher average levels of alcohol consumption than countries with more traditional values, such as hierarchy and being part of a collective. This new ...

A walk at the mall or the park? New study shows, for moms and daughters, a walk in the park is best

November 17, 2017
Spending time together with family may help strengthen the family bond, but new research from the University of Illinois shows that specifically spending time outside in nature—even just a 20-minute walk—together can ...

Risk of distracted driving predicted by age, gender, personality and driving frequency

November 17, 2017
New research identifies age, gender, personality and how often people drive as potential risk factors for becoming distracted while driving. Young men, extroverted or neurotic people, and people who drive more often were ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

androloma
not rated yet Oct 21, 2014
I'm sure there are many people unacquainted with this subject who should know all about vastly differing findings. Because that's the data they want to believe in. Something about a loss of exactly 8 IQ points in users.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.