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By Dominic Hughes Health correspondent, BBC News
Young people who smoke cannabis for years run the risk of a significant and
irreversible reduction in their IQ, research suggests.
The findings come from a study of around 1,000 people in New Zealand.
An international team found those who started using cannabis below the age of
18 - while their brains were still developing - suffered a drop in IQ.
A UK expert said the research might explain why people who use the drug often
seem to under-achieve.
For more than 20 years researchers have followed the lives of a group of people
from Dunedin in New Zealand.
They assessed them as children - before any of them had started using cannabis
- and then re-interviewed them repeatedly, up to the age of 38.
Having taken into account other factors such as alcohol or tobacco dependency
or other drug use, as well the number of years spent in education, they found
that those who persistently used cannabis - smoking it at least four times a
week year after year through their teens, 20s and, in some cases, their 30s -
suffered a decline in their IQ.
The more that people smoked, the greater the loss in IQ.
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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
Professor Terrie Moffitt Institute of Psychiatry, King's College London
The effect was most marked in those who started smoking cannabis as
adolescents.
For example, researchers found that individuals who started using cannabis in
adolescence and then carried on using it for years showed an average
eight-point IQ decline.
Stopping or reducing cannabis use failed to fully restore the lost IQ.
The researchers, writing in the US journal Proceedings of the National Academy
of Sciences, found that: "Persistent cannabis use over 20 years was associated
with neuropsychological decline, and greater decline was evident for more
persistent users."
"Collectively, these findings are consistent with speculation that cannabis use
in adolescence, when the brain is undergoing critical development, may have
neurotoxic effects."
One member of the team, Prof Terrie Moffitt of King's College London's
Institute of Psychiatry, said this study could have a significant impact on our
understanding of the dangers posed by cannabis use.
"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.
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There are a lot of clinical and educational anecdotal reports that cannabis
users tend to be less successful in their educational achievement, marriages
and occupations
Professor Robin Murray Instuitute of Psychiatry, King's College London
BBC Health: Cannabis in depth
"Participants were frank about their substance abuse habits because they trust
our confidentiality guarantee, and 96% 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."
Robin Murray, professor of psychiatric research, also at the King's College
London Institute of Psychiatry but not involved in the study, said this was an
impressive piece of research.
"The Dunedin sample is probably the most intensively studied cohort in the
world and therefore the data are very good.
"Although one should never be convinced by a single study, I take the findings
very seriously.
"There are a lot of clinical and educational anecdotal reports that cannabis
users tend to be less successful in their educational achievement, marriages
and occupations.
"It is of course part of folk-lore among young people that some heavy users of
cannabis - my daughter callers them stoners - seem to gradually lose their
abilities and end up achieving much less than one would have anticipated. This
study provides one explanation as to why this might be the case.
"I suspect that the findings are true. If and when they are replicated then it
will be very important and public education campaigns should be initiated to
let people know the risks."