Persistent cannabis use among teenagers under 18 years of age results in neuropsychological decline, which persists even after they stop smoking, researchers from the USA and UK reported in Proceedings of the National Academies of Sciences. The authors added that the decrease in IQ did not seem to occur among persistent cannabis users who started after the age of 18.
Persistent cannabis use means daily pot smoking.
They found that early-onset regular pot users had IQs 8 points lower than their counterparts who never smoked or started after they were 18 years of age.
The authors explained that as progressively fewer teenagers today believe cannabis harms health, the age at which people start using it on a daily basis has been dropping.
Study leader, Madeline Meier, a post-doctoral researcher at Duke University in Durham, North Carolina, said that persistent cannabis use among American high school students is higher than it has ever been, according to the National Institute on Drug Abuse.
Doctor Meier and Professor Terrie Moffitt, at King’s College London’s Institute of Psychiatry, and researchers from the University of Otago, New Zealand, set out to determine whether there might be a link between persistent cannabis use among teenagers and neuropsychological decline. They also wanted to find out whether the neuropsychological decline persisted after people stopped smoking cannabis.
They examined data from members of the Dunedin study, involving 1,037 New Zealanders who were followed from the day they were born until they were 38. Their cannabis usage was determined at the ages of 18, 21, 26, 32 and 38. They were tested for intelligence, memory and attention initially when they were 13 years old, before cannabis use began, and again when they were 38.
They found that the neuropsychological effect of persistent cannabis smoking was most damaging if it started before the user was 18 years of age, and much less so if onset began later.
Heavy and regular cannabis use was linked to neuropsychological decline across virtually all domains of functioning, even after taking into account many years of education.
The authors noted that cognitive problems were greater among persistent cannabis users. Most of the cognitive impairment occurred in those who started smoking pot regularly during their pre-18 years.
Even after stopping cannabis use, neuropsychological deficits were never recovered among those who had started smoking earlier during their teenage years.
The authors concluded:
“Findings are suggestive of a neurotoxic effect of cannabis on the adolescent brain and highlight the importance of prevention and policy efforts targeting adolescents.”
The researchers emphasized that it is not possible to determine, after reading about this study, what the safest age to start pot smoking might be.
Previous studies have linked marijuana usage to health harms as well as benefits:
- An Australian study published in Archives of General Psychiatry in 2009 found that prolonged cannabis use was associated with a higher psychosis risk among young adults who had started in their teenage years.
- Researchers at The Zucker Hillside Hospital in Long Island, Mount Sinai School of Medicine and the Albert Einstein College of Medicine in New York found that cannabis enhances bipolar disorder patients’ neurocognitive performance. Their study was published in Psychiatry Research in August 2012.
- A study carried out by a team from the University of Bristol, England, found that cannabis causes “chaos in the brain”; brain activity becomes inaccurate and uncoordinated, leading to schizophrenia-like behavioral and neurophysiological impairments. Their study was published in the October 2011 issue of Neuroscience.
Written by Christian Nordqvist