Cannabis Use In Teens Used To Link Drop In IQ

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 said: "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."

(CD)