Teenage cannabis use reduces IQ permanently
Study finds marijuana is 'safe' for over-18s but 'risky' for younger users
THE LATEST results from a major scientific study ongoing since 1972 show that smoking cannabis as a teenager can permanently lower your IQ. But the international team of scientists also found that smoking the drug was 'safe' for over-18s.
Prof Terrie Moffitt, of the international group of scientists who conducted the study, said they had ruled out other factors such as alcohol use, dropping out of school or use of other drugs and were sure the IQ loss was related to cannabis alone.
She told the BBC the effects included loss of concentration, speed of mental reactions and problems with memory. They appeared to remain in place even if the subjects stopped smoking the plant's leaves or resin for a year.
But the effects were only found to be irreversible in those who had started smoking the drug as teenagers: Moffitt said those who had started later in life suffered the same problems but recovered if they stopped using the drug.
She speculated that the problem might be because the young people's brains were still growing. The study was started when its 1,000 test subjects, from Dunedin in New Zealand, were just children and had not started using cannabis.
It continued through their teenage years and up until they reached the age of 38. Individuals who started using cannabis as adolescents and continued smoking it for years suffered an average decline of eight IQ points.
Moffitt, of King's College, London, said: "This work took an amazing scientific effort. We followed almost 1,000 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 per cent of the original participants stuck with the study from 1972 to today.
"It is such a special study that I'm fairly confident that cannabis is safe for over-18 brains, but risky for under-18 brains."