"We have to be clear about getting the message out that marijuana isn't really a benign substance," she said. "It has a direct effect on executive function. The earlier you begin using it, and the more you use of it, the more significant that effect."
The study included 33 chronic marijuana smokers and 26 control subjects who did not smoke marijuana. They were given a battery of neurocognitive tests assessing executive function, including the Wisconsin Card Sorting Test, which involves sorting different cards based on a set of rules given. During the test, the rules are changed without warning and subjects must adjust their responses to the new rules.
The findings showed habitual marijuana users made repeated errors even when told that they were wrong. Users also had more trouble maintaining a set of rules, suggesting an inability to maintain focus. Early onset users and those who used the most marijuana had the most trouble with the test, making more than twice as many errors and fewer correct responses than later onset smokers."