A paper presented at the annual congress of the European College of Neuropsychopharmacology (ECNP) in Berlin says that moderate cannabis use by adolescents does not lead to educational or intellectual decline, 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. They analyzed 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 cannabis use. 

Their method has some limitations: cannabis use was self-reported and the measure of IQ taken at age 15 was an abbreviated version of the standard Wechsler IQ test, among other things.  

The researchers used regression analysis to look at how cannabis use affected both intellectual and educational performance. 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 children 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 cannabis users may have resulted from the lifestyle, behavior 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."