There is no connection between occasional adolescent cannabis use in adolescents and IQ or exam results, a large UK-based study has found.
Although the research shows moderate cannabis use does not lead to poorer education and intellectual performance, heavy marijuana use is associated with slightly poorer exam results at the age of 16.
The study showed cannabis use was highly correlated with other behaviours such as drinking alcohol, smoking cigarettes or participating in other drug use.
Lead researcher Claire Mokrysz, of the University College London, said: "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, behaviour and personal history typically associated with cannabis use, rather than cannabis use itself."
For the study, the researchers used data from the Avon Longitudinal Study of Parents and Children, which follows the health of children born in the Bristol area in 1991 and 1992.
The researchers analysed data from 2,612 children who had their IQ tested at the age of eight and again at 15, when each person in the study completed a survey on cannabis use. Analysts used regression analysis to examine how marijuana use affected their educational performances.
The results showed that cannabis use appeared to be associated with decreased intellectual performance. However, it was highly correlated with other risky behaviours such as alcohol, cigarette and drug use.
With these factors taken into account, the researchers found there was no relationship between cannabis use and lower IQ at the age of 15.
Heavier cannabis users – who used the drug at least 50 times by age 15 - did show marginally impaired educational abilities.
These children tended to have poorer exam results, around 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.
Important health message about cannabis
"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," Mokrysz explained.
"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 is a potentially important public health message," she added. "The belief that cannabis is particularly harmful may detract focus from and awareness of other potentially harmful behaviours."
The findings will be presented at the European College of Neuropsychopharmacology annual congress in Berlin.
The chair of the annual congress, Guy Goodwin, from the University of Oxford, told BBC News: "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."