Pubdate: Tue, 28 Aug 2012
Source: Irish Times, The (Ireland)
Copyright: 2012 The Irish Times
Author: John von Radowitz


CANNABIS can lower the IQ of young teenagers and may cause permanent 
mental impairment, research has shown.

The most persistent users suffer an average eight- point decline in 
IQ between adolescence and adulthood, according to the study of more 
than 1,000 participants.

Quitting or cutting down on cannabis later in life did not fully 
reverse the impact on those who started taking the drug in their early teens.

But the study found no evidence of similar problems affecting people 
who only took up cannabis as adults.

The international team, led by US psychologist Dr Madeline Meier from 
Duke University in Durham, Carolina, said: "Impairment was 
concentrated among adolescent- onset cannabis users, with more 
persistent use associated with greater decline.

"Further, cessation of cannabis use did not fully restore 
neuropsychological functioning among adolescent onset cannabis users."

Scientists followed the progress of participants from birth to the 
age of 38, carrying out neuropsychological tests at age 13 and again 
at the end of the study. Cannabis use was recorded at five intervals 
from the age of 18 onwards.

The tests showed wide- ranging mental declines among men and women 
who began taking cannabis at a young age and continued using the drug 
regularly for more than 20 years.

"The most persistent adolescent- onset cannabis users evidenced an 
average eight- point IQ decline from childhood to adulthood," said 

Persistent cannabis use appeared to affect everyday mental functioning.

Users experienced significant attention and memory problems, 
according to friends and relatives questioned by the researchers.

While quitting may prevent further impairment, it did not appear to 
restore normal mental functioning for those whose cannabis habit 
began in adolescence, said the scientists.

Puberty was a period of "critical brain development" when neural 
circuits were still forming, they pointed out. At this stage in life 
the brain was vulnerable to "toxic insult".

Professor Terrie Moffitt, from the Institute of Psychiatry at King's 
College London, who took part in the study, said: "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." 
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