Investigators at the University College of London analyzed data from 2,612 subjects who had their IQ tested at the age of eight and again at age 15. They REPORTED no relationship between cannabis use and lower IQ at age 15  when confounding factors such as subjects’ history of alcohol use and cigarette use were taken into account.
“In particular alcohol use was found to be strongly ASSOCIATED with IQ decline,” the authors wrote in a press release cited  by The Washington Post. “No other factors were found to be predictive of IQ change.”
Quoted in the Independent BUSINESS Times, the study’s lead author 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, behavior and personal history typically associated with cannabis use, rather than cannabis use itself.”
The investigators acknowledged that more chronic marijuana use, defined in the study as a subject’s admission of having consumed cannabis 50 times or more by age 15, was correlated with slightly poorer exam results at the age of 16 — even after controlling for other variables. However, investigators admitted : “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?”
Commenting on the newly presented data, the meeting’s Chair, 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.”
In a recent review published in the New England Journal of Medicine, the NIDA Director Nora Volkow alleged  that cannabis use, particularly by adolescents, is ASSOCIATED with brain alterations and lower IQ. However, the IQ study  cited by Ms. Volkow as the basis of her claim was later questioned in a separate analysis published in the Proceedings of the National Academy of Sciences. That paper suggested  that socioeconomics, not subjects’ cannabis use, was responsible for differences in IQ and that the plant’s “true effect [on intelligence quotient] could be zero.”
A previous assessment  of cannabis use and its potential impact on intelligence quotient in a cohort of young people tracked since birth REPORTED, “[M]arijuana does not have a long-term negative impact on global intelligence.”