13:33 28 August 2012
A study has found that people who smoke cannabis on a regular basis as a teenager could permanently affect their brain.
Over 1,000 people participated in the research in New Zealand, which looked at adults aged 38 who suffered from the effects of the drug use from 20 years earlier.
It found that those who used cannabis were more likely to end up scoring lower on an IQ test than those people who had not smoked it.
It is thought intelligence, attention span and memory are all affected by the drug.
"The people who used pot persistently as teens scored significantly worse on most of the tests," Madeline Meier, of Duke University in North Carolina, told The Independent.
"Friends and relatives routinely 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."
Scientists believe the long-term effects of the drug relates to the impact it has on the brain as it is still developing in people’s teenage years of their life.
Research shows that weekly cannabis use before the age of 18 saw an average loss of eight points in an IQ test.
“Higher IQs correlate with higher education and income, better health and a longer life,” Dr Meier added, as quoted by The Guardian.
"Somebody who loses eight IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come.”
The participants were tested for their intelligence before they had tried the drug and then again 25 years later.
Another author of the study Professor Terrie Moffitt, of King's College London's Institute of Psychiatry, told The Guardian: "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."
A loss of eight points in an IQ test could result in a person dropping from a category shared by half of the population, to a lower category which is shared by only 29% of the population.
Approximately 5% of the study used cannabis at least once a week during their teenage years or depended on the drug.
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