Teenagers who regularly smoke cannabis are putting themselves at risk of permanently damaging their intelligence, according to a landmark study.
Researchers found persistent users of the drug, who started smoking it at school, had lower IQ scores as adults.
They were also significantly more likely to have attention and memory problems in later life, than their peers who abstained.
Furthermore, those who started as teenagers and used it heavily, but quit as adults, did not regain their full mental powers, found academics at King’s College London and Duke University in the US.
They looked at data from over 1,000 people from Dunedin in New Zealand, who have been followed through their lives since being born in 1972 or 1973.
Participants were asked about cannabis usage when they were 18, 21, 26, 32 and 38. Their IQ was tested at 13 and 38. In addition, each nominated a close friend or family member, who was asked about attention and memory problems.
About one in 20 admitted to starting cannabis use before the age of 18, while a further one in 10 took up the habit in the early or mid 20s.
Professor Terrie Moffitt, of KCL’s Institute of Psychiatry, who contributed to the study, published in the journal Proceedings of the National Academy of Sciences, said “persistent users” who started as teenagers suffered a drop of eight IQ points at the age of 38, compared to when they were 13.
Persistent users meant those who used it during at least three of the ages from 18 to 38, and who said at each occasion they were smoking it on at least four days a week.
She said: “Adolescent-onset cannabis users, but not adult-onset cannabis users, showed marked IQ decline from childhood to adulthood.
“For example, individuals who started using cannabis in adolescence and used it for years thereafter showed an average eight-point IQ decline.
“Quitting or reducing cannabis use did not appear to fully restore intellectual functioning among adolescent-onset former persistent cannabis users,” she said.
Although eight points did not sound much, it was not trivial, she warned.
It meant that an average person dropped far down the intelligence rankings, so that instead of 50 per cent of the population being more intelligent than them, 71 per cent were.
“Research has shown that IQ is a strong determinant of a person’s access to a college education, their lifelong total income, their access to a good job, their performance on the job, their tendency to develop heart disease, Alzheimer’s disease, and even early death,” she said.
“Individuals who lose eight IQ points in their teens and 20s may be disadvantaged, relative to their same-age peers, in most of the important aspects of life and for years to come.”
The cognitive abilities of the 10 per cent of people who started in their 20s - who could loosely be classed as college smokers - also suffered while they were still smoking.
However, if they gave up at least a year before their IQ test at 38, their intelligence recovered, suggesting their brains were more resilient and bounced back.
Prof Moffitt said adolescent brains appeared "more vulnerable to damage and disruption" from cannabis than those of fully mature adults.
Reliable figures on cannabis usage among today’s British teens and twentysomethings are hard to come by.
But Prof Moffitt said there was growing concern in the US that cannabis was increasingly being seen as a safe alternative to tobacco.
“This is the first year that more secondary school students in the US are using cannabis than tobacco, according to the Monitoring the Future project at the University of Michigan,” she noted.
“Fewer now think cannabis is damaging than tobacco. But cannabis is harmful for the very young.”
Pot is a god send to people with cancer, but like all drugs it has side effects.
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