It's not exactly news that smoking pot dims the mental faculties, at least at the time. Regular users do tend to be slow of speech and mind.
A new study apparently finds that starting young tends to make the damage permanent, costing the user about 8 IQ points in adulthood.
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.
On the other hand, if you wait until age 20, when the brain has developed, decreased mental function appears to be temporary and no significant permanent damage is observed. But I suspect that most tokers start early.