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Smoking marijuana does not have a long-term effect on intelligence, say researchers in Canada who have followed volunteers from before birth to early adulthood.
Heavy pot smokers did experience a dip in their intelligence quotient (IQ). But people who had once smoked heavily and then given up were right back up to normal, the study found. Light smokers appeared no different from non-smokers.
What the researchers do not know is if decades of pot-smoking could have a more lasting impact. Looking at long-term users in their 30s or 40s could show different results, admits Peter Fried, at Carleton University in Ottawa, who led the study. "Perhaps the nervous system isn't as flexible then," he says.
"You can't argue with what they're saying," says William Campbell, President of the Canadian Society of Addiction Medicine. "It doesn't surprise me or disappoint me."
Urine test
Fried and his team followed 70 middle-class kids from the womb. IQ tests were taken at around age 10 and then again between 18 and 20. As adults, the participants were asked about a range of behaviours, including pot smoking. They also under went urine analysis to check their answers.
At the time of the second questionnaire, nine had been heavy users in the past but had not smoked for over three months. Fifteen were still smoking cannabis heavily - at least five joints a week and nine were current light users who smoked a few joints weekly. The rest of the volunteers had never been regular users, so had either never smoked marijuana or had done so less than once a week.
Only the heavy current users had experienced a decline in their IQ scores over the 10-year period - about four points. Light users, former users and abstainers all saw their IQ scores climb between two and six points.
Fried concedes that while IQ may be spared, memory and attention may be harder hit and is examining the effect now: "The most-often stated reason for quitting was they felt their short-term memory was affected."
Source: Marijuana does not dent IQ permanently - 08 April 2002 - New Scientist
Heavy pot smokers did experience a dip in their intelligence quotient (IQ). But people who had once smoked heavily and then given up were right back up to normal, the study found. Light smokers appeared no different from non-smokers.
What the researchers do not know is if decades of pot-smoking could have a more lasting impact. Looking at long-term users in their 30s or 40s could show different results, admits Peter Fried, at Carleton University in Ottawa, who led the study. "Perhaps the nervous system isn't as flexible then," he says.
"You can't argue with what they're saying," says William Campbell, President of the Canadian Society of Addiction Medicine. "It doesn't surprise me or disappoint me."
Urine test
Fried and his team followed 70 middle-class kids from the womb. IQ tests were taken at around age 10 and then again between 18 and 20. As adults, the participants were asked about a range of behaviours, including pot smoking. They also under went urine analysis to check their answers.
At the time of the second questionnaire, nine had been heavy users in the past but had not smoked for over three months. Fifteen were still smoking cannabis heavily - at least five joints a week and nine were current light users who smoked a few joints weekly. The rest of the volunteers had never been regular users, so had either never smoked marijuana or had done so less than once a week.
Only the heavy current users had experienced a decline in their IQ scores over the 10-year period - about four points. Light users, former users and abstainers all saw their IQ scores climb between two and six points.
Fried concedes that while IQ may be spared, memory and attention may be harder hit and is examining the effect now: "The most-often stated reason for quitting was they felt their short-term memory was affected."
Source: Marijuana does not dent IQ permanently - 08 April 2002 - New Scientist
