Smoking cannabis can permanently lower the IQ of teenagers who use it, a new study today reveals.
The most persistent young users of the Class B drug suffer an average eight-point drop in IQ between adolescence and adulthood.
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That can mean the difference between being of average intelligence, or in the so-called 50th percentile, and being in the 29th percentile.
Users also have “significantly” more attention and memory problems than non-users – regardless of educational background, use of alcohol or intake of other drugs.
Meanwhile the report also finds that quitting or cutting back on marijuana use later in life does not fully reverse the impact.
The effect on teens is marked: there is no evidence of similar effects on people who only take up cannabis as adults, leading experts to deduce that the drug may disrupt vital developing brain circuits during puberty.
The international study, led by US psychologist Dr Madeline Meier from Duke University in Carolina, focused on more than 1,000 cannabis users over a period of many years.
Dr Meier said: “Persistent cannabis use was associated with neuropsychological decline broadly across domains of functioning even after controlling for years of education.
"Impairment was concentrated among adolescent-onset cannabis users, with more persistent use associated with greater decline. Further, cessation of cannabis use did not fully restore neuropsychological functioning among adolescent-onset cannabis users."
Professor Terrie Moffitt, from the Institute of Psychiatry at King's College London, who also took part in the study, added: "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.
“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."
Her King’s colleague Professor Robin Murray said: "We have known for some time that heavy use of cannabis increases risk of schizophrenia-like psychoses but this remains a relatively rare outcome.
"There are far fewer studies on its effect on minor psychiatric illness or on everyday life. However, there are a lot of clinical and educational anecdotal reports that cannabis users tend to be less successful in their educational achievement, marriages and occupations.
"It is of course part of folk-lore among young people that some heavy users of cannabis - my daughter callers them 'stoners' - seem to gradually lose their abilities and end up achieving much less than one would have anticipated.
“This study provides one explanation as to why this might be the case."
When the participants’ IQ tests at age 13 and age 38 were compared, those who started using the drug in their teens showed an average decline in IQ of eight points. Quitting did not reverse the loss.
An eight-point loss is not insignificant, claims the study, pointing out that an IQ of 100 to 92 represents a drop from being in the 50th percentile to being in the 29th.
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Higher IQ also correlates with higher education and income, better health and a longer life.
Friends and relatives interviewed as part of the study were more likely to report that the persistent cannabis users had attention and memory problems such as losing focus and forgetting to do tasks.
Concludes Meier: "Marijuana is not harmless, particularly for adolescents.”