Young, healthy people who smoke fewer than five marijuana cigarettes a week don't seem to suffer any long-term effects to their IQ, Canadian researchers have found.

Heavy marijuana use – defined as smoking more than five joints a week – did seem to hurt IQ by an average of four points.

But the researchers also found the decline in IQ appeared to recover when the user quit smoking.

Carleton University psychology Prof. Fred Fried and his colleagues followed 70 children and their mothers who enrolled in the Ottawa Prenatal Prospective Study.

The children had their IQs measured when they were between 9 and 12 years old, before any of them started using marijuana.

The subjects, who are now 17 to 20 years old, also had their IQs checked after they began smoking marijuana. Urine tests were done to measure how much the smokers were exposed to.

About 50 studies have looked at the long-term effects of marijuana, but researchers have always had trouble deciding what standard to use. Usually researchers look at the effects of drug use after just a few days.

Fried's study is one of the first to look at Intelligence Quotient levels before children take up the habit, which offers a natural comparison point.

However, Fried acknowledged that his study only looked at IQ, which he said is a relatively insensitive measure of cognition.

Fried and his colleagues plan to look at more subtle effects on memory, attention and processing speed in about twice as many subjects in the future.

And he cautions it isn't clear whether the results apply to older individuals who have smoked for longer periods of time.

Fried's study appears in Tuesday's issue of the Canadian Medical Association Journal.