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Teenage cannabis use linked to decline in IQ

Posted on 28/08/2012
Teenager smoking

New research finds that persistent cannabis use during adolescence can cause lasting harm to a person's intelligence, attention and memory. 

Researchers from the Medical Research Council (MRC) Social Genetic and Development Psychiatry Centre at King’s College London’s Institute of Psychiatry (IoP) collaborated with scientists from Duke University in the USA and the University of Otago in New Zealand on the study published today in Proceedings of the National Academy of Sciences. They found that individuals who started using cannabis in adolescence and used it more than once a week for years afterwards showed an average decline in IQ of 8 points when their age 13 and age 38 IQ tests were compared. 

The team studied the association between persistent cannabis use and neuropsychological functioning in a group of 1,037 individuals (from the Dunedin Multidisciplinary Health and Development Study) followed from birth in 1972/1973 to age 38. Cannabis use was ascertained in interviews at ages 18, 21, 26, 32 and 38. Neuropsychological testing to assess memory, processing speed, reasoning and visual processing was conducted at age 13, before cannabis use, and again at age 38. 

About 15 percent of the study group were considered persistent cannabis users and 5 percent were using cannabis more than once a week before age 18.  Individuals who started using cannabis persistently during adolescence a greater decline in neuropsychological function that those who started in adulthood.

Persistent cannabis use was associated with broad impairment across five domains of neuropsychological functioning, and remained significant even after controlling for years of education and use of other drugs including alcohol. Quitting or reducing cannabis use did not fully restore neuropsychological functioning among individuals who started using cannabis in adolescence.

Professor Terrie Moffitt, lead author, who holds a dual appointment at King’s and Duke says: "This work took an amazing scientific effort. We followed almost 1000 participants, we tested their mental abilities as kids before they ever tried cannabis, and we tested them again 25 years later after some participants became chronic users. Participants were frank about their substance abuse habits because they trust our confidentiality guarantee and 96% of the original participants stuck with the study from 1972 to today. It’s such a special study that I'm fairly confident that cannabis is safe for over-18 brains, but risky for under-18 brains."

Dr Madeline H. Meier, lead researcher from Duke University, says: “Our study shows that adolescents are particularly vulnerable to the effects of cannabis. The brain undergoes important maturational changes during adolescence. For example, it is thought that certain developmental brain changes that occur during adolescence make the brain more efficient.  Adolescents who use cannabis may disrupt these critical maturational processes.”

Dr Avshalom Caspi, co-author, also from King’s and Duke, says: “I hope this research does not get mired in debates about marijuana legalization for adults.  The simple message is that substance use is not healthy for kids.  That's true for tobacco, alcohol, and apparently now for cannabis.”

Professor Hugh Perry, Chairman of the MRC's Neuroscience and Mental Health Board commented on the study: "Previous studies have suggested that adolescents may be particularly vulnerable to the effects of cannabis on IQ, however, this careful research provides much stronger evidence as it rules out the possibility that poorer test performance among adolescent-onset cannabis is due to other factors that predates their cannabis-use. This is important and valuable knowledge for public health as is part of MRC's research strategy in mental health and addiction to support the highest quality research to untangle the complex relationship between early life factors and their consequences for mental wellbeing in later life."

The Dunedin Multidisciplinary Health and Development Research Unit is supported by the New Zealand Health Research Council. The research received support from the UK Medical Research Council (MRC), the US National Institute on Aging and the US National Institute on Drug Abuse. Additional support was provided by the Jacobs Foundation. 

For full paper: Meier, M.H. et al. ‘Persistent cannabis users show neuropsychological decline from childhood to midlife’, Proceedings of the National Academy of Sciences  (August 2012) doi: 10.1073/pnas.1206820109

For further information, please contact Seil Collins, Press Officer, Institute of Psychiatry, King’s College London, tel: 0207 848 5377 or email:

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