NEW YORK (Reuters Health) - Heavy marijuana use can dull a young person's IQ, but the effects may not be long-lasting, a small study suggests.

Canadian researchers found that young adults who currently smoked five or more joints a week showed it on IQ tests. However, the same effects were not seen in those who used to smoke heavily but had quit, according to findings published in a recent issue of the Canadian Medical Association Journal.

The 74 study participants were part of a larger study that had followed them since birth. All had taken IQ tests between the ages of 9 and 12 and again between ages 17 and 20. They also reported on their drug use and had urine tests to verify recent use.

Results indicate that only current heavy pot smokers showed a decline in IQ score since their preteen years, according to Dr. Peter Fried and colleagues at Carleton University in Ottawa, Ontario. The same was not true of less frequent marijuana users or those who had been abstinent for at least 3 months.

The link between heavy current use and IQ decline remained even when the researchers accounted for other factors such as past school performance, use of other drugs and parents' education, the investigators found.

Many studies have shown that marijuana use can lead to short-term mental deficits, but whether the effects last is debatable. One recent study suggested that long-time, heavy pot smokers may see lasting problems with memory and attention span.

And Fried's team notes that specific mental functions like memory and attention may be more vulnerable to marijuana use than the broad measure of intelligence this study focused on.

They add that both their finding of an IQ effect among current heavy users and a lack of one among former users should be viewed "cautiously," in part because of the study size. Only 15 participants currently smoked heavily, and nine were former marijuana users.

SOURCE: Canadian Medical Association Journal 2002;166:887-891.

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