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Jackson NJ, Isen JD, Khoddam R, Irons D, Tuvblad C, Iacono WG, McGue M, Raine A, Baker LA. 
“Impact of adolescent marijuana use on intelligence: Results from two longitudinal twin studies”. 
Proc Natl Acad Sci U S A. 2016 Feb 03;113(5):E500-8.
Marijuana is one of the most commonly used drugs in the United States, and use during adolescence--when the brain is still developing--has been proposed as a cause of poorer neurocognitive outcome. Nonetheless, research on this topic is scarce and often shows conflicting results, with some studies showing detrimental effects of marijuana use on cognitive functioning and others showing no significant long-term effects. The purpose of the present study was to examine the associations of marijuana use with changes in intellectual performance in two longitudinal studies of adolescent twins (n = 789 and n = 2,277). We used a quasiexperimental approach to adjust for participants' family background characteristics and genetic propensities, helping us to assess the causal nature of any potential associations. Standardized measures of intelligence were administered at ages 9-12 y, before marijuana involvement, and again at ages 17-20 y. Marijuana use was self-reported at the time of each cognitive assessment as well as during the intervening period. Marijuana users had lower test scores relative to nonusers and showed a significant decline in crystallized intelligence between preadolescence and late adolescence. However, there was no evidence of a dose-response relationship between frequency of use and intelligence quotient (IQ) change. Furthermore, marijuana-using twins failed to show significantly greater IQ decline relative to their abstinent siblings. Evidence from these two samples suggests that observed declines in measured IQ may not be a direct result of marijuana exposure but rather attributable to familial factors that underlie both marijuana initiation and low intellectual attainment.
Comments and Responses to this Article
Oct 14, 2016 16:02
NIDA Appears to Abandon 50 Years of Marijuana Damages IQ #

It appears that the US National Institute on Drug Abuse, the main funder of studies about psychoactive drugs and the primary anti-drug data source seems as if it is finally abandoning it's long held, unsubstantiated claim that smoking pot causes damage to the IQ.

This matched-twin longitudinal study was unable to confirm any causal linkage. The authors did find an association between heavy cannabis use and reduced learning in highschool. But, after adjusting for other factors, the association disappeared.

What does this all mean? The main thing to remember is that while cannabis use does not seem to cause any brain damage, it can take up a lot of time and make people less interested in academic performance. If you want to do anything other than smoke pot, you might want to do something other than smoke pot.
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