FRIDAY, April 5, 2002 (HealthDayNews) -- If you smoke fewer than five joints a week, your IQ should stay intact. However, if you toke more, your IQ could go up in smoke.
A new Canadian study has found that heavy pot use decreases a person's IQ by an average of four points, while the occasional joint doesn't seem to damage intelligence.
But the study also found that if you stop smoking marijuana for at least three months, your IQ may benefit: Former users gained an average of 3.5 IQ points after they quit.
"The most intriguing finding, certainly, is the recovery of function in these individuals," says study author Peter Fried, a professor of psychology at Carleton University in Ottawa.
This is the first study that actually measures the intelligence of people before and after they started using marijuana. It is published in the current issue of the Canadian Medical Association Journal.
Fried and his colleagues studied 70 people and compared their IQ scores from when they were 9 to 12 years old -- before any started using marijuana -- to their IQ scores when they were 17 to 20 years old.
The researchers found the IQ scores of the heavy marijuana users decreased by 4.1 points on average, while light users had an average gain of 5.8 points, former users showed an average increase of 3.5 points, and nonusers had an average increase of 2.6 points.
While it may appear that the findings show light users have higher IQ increases than nonusers, Fried says that the people in the study were middle class and
that sort of IQ increase is expected and "perfectly normal" for them during
those years. It has no connection to whether or not they used pot, he adds.
The group included 37 people who never used marijuana, nine light users who smoked less than five joints a week, 15 heavy users who smoked more than five joints a week, and nine former users who'd shunned marijuana for at least three months.
The people in the marijuana study are part of the Ottawa Prenatal Prospective Study, which started in 1978. Pregnant women enrolled in that study and their children have been monitored since.
Data collected from the study let Fried make the before-and-after IQ comparisons.
As a standard of measurement, Fried notes that an IQ score of 100 is considered average; students with an IQ no higher than 77 are generally offered special education.
And he adds that a four-point shift in IQ wouldn't really be apparent.
"If you were walking down the street, you wouldn't be able to tell if somebody had an IQ of 104 or 108," he says.
Although his study indicates your IQ can recover when you quit smoking weed, Fried emphasizes it may not apply to a 50-year-old reformed pothead who smoked dope for decades.
"These are 17- to 20-year-olds and they've only been smoking, on average, for three years. I don't know if we would see a recovery of function in folks who were older, and who had perhaps been smoking for 15 to 20 years," he says.
Fried adds this study doesn't tell the whole story about how marijuana affects your brain.
"IQ is a relatively insensitive measure of drug effects. Thus, I do not know whether things like memory or attention or (information-)processing speed would in fact recover," Fried says.
He offers an interesting aside about the longtime marijuana users in this study who gave up the habit.
"When we asked them why they quit, the most common statement was that they felt it impacted negatively on their memory," Fried says.
This study offers important new information about the effects of marijuana, says Richard Garlick, the director of communications for the Canadian Centre on Substance Abuse.
"I think what this study does is give us one more piece of information on what has been a somewhat cloudy aspect of cannabis use," Garlick says.
He says research seems to show marijuana isn't physically addictive. Advocates who want marijuana legalized say that proves it's benign.
However, Garlick says that argument overlooks other potential dangers, like the effect on IQ noted in this study.
"It's safe to say that anybody in this age group who's consuming at a rate of five joints a week, according to this study, is having some reduction in IQ, and therefore [in] their ability to learn and absorb information. It is something we should be paying attention to and be concerned about," Garlick says.
Almost 69 million Americans over the age of 12 have tried marijuana at least once, says the U.S. National Institute on Drug Abuse. Marijuana is the most frequently used illegal drug in the United States.
What To Do
NIDA has information about marijuana for teenagers and for parents.
